A hard disk drive ( HDD ),

commonly referred to as a hard drive, hard disk or fixed disk drive,[1] is a non-

volatile storage device which stores digitally encoded data on rapidly rotating

platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct

from its medium, such as a tape drive and its tape, or a floppy disk drive and its

floppy disk. Early HDDs had removable media; however, an HDD today is typically a

sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.[2]

A HDD is a rigid-disk drive, although it is probably never referred to as such. By way

of comparison, a so-called "floppy" drive (more formally, a diskette drive) has a disc

that is flexible. Originally, the term "hard" was temporary slang, substituting "hard"

for "rigid", before these drives had an established and universally-agreed-upon

name. Some time ago, IBM's internal company term for an HDD was "file".

HDDs (introduced in 1956 as data storage for an IBM accounting computer[3]) were

originally developed for use with general purpose computers; see History of hard

disk drives.

In the 21st century, applications for HDDs have expanded to include digital video

recorders, digital audio players, personal digital assistants, digital cameras and

video game consoles. In 2005 the first mobile phones to include HDDs were

introduced by Samsung and Nokia.[4] The need for large-scale, reliable storage,

independent of a particular device, led to the introduction of configurations such as

RAID arrays, network attached storage (NAS) systems and storage area network

(SAN) systems that provide efficient and reliable access to large volumes of data.

Note that although not immediately recognizable as a computer, all the

aforementioned applications are actually embedded computing devices of some

sort.

Technology

HDDs record data by magnetizing ferromagnetic material directionally, to represent

either a 0 or a 1 binary digit. They read the data back by detecting the magnetization

of the material. A typical HDD design consists of a spindle which holds one or more

flat circular disks called platters, onto which the data is recorded. The platters are

made from a non-magnetic material, usually aluminum alloy or glass, and are coated

with a thin layer of magnetic material. Older disks used iron(III) oxide as the

magnetic material, but current disks use a cobalt-based alloy.

A cross section of the magnetic surface in action. In this case the binary data is encoded

using frequency modulation:

The platters are spun at very high speeds (details follow). Information is written to a

platter as it rotates past devices called read-and-write heads that operate very close

(tens of nanometers in new drives) over the magnetic surface. The read-and-write

head is used to detect and modify the magnetization of the material immediately

under it. There is one head for each magnetic platter surface on the spindle,

mounted on a common arm. An actuator arm (or access arm) moves the heads on

an arc (roughly radially) across the platters as they spin, allowing each head to

access almost the entire surface of the platter as it spins. The arm is moved using a

voice coil actuator or (in older designs) a stepper motor. Stepper motors were

outside the head-disk chamber, and preceded voice-coil drives. The latter, for a

while, had a structure similar to that of a loudspeaker; the coil and heads moved in a

straight line, along a radius of the platters. The present-day structure differs in

several respects from that of the earlier voice-coil drives, but the same interaction

between the coil and magnetic field still applies, and the term is still used.

Older drives read the data on the platter by sensing the rate of change of the

magnetism in the head; these heads had small coils, and worked (in principle) much

like magnetic-tape playback heads, although not in contact with the recording

surface. As data density increased, read heads using magnetoresistance (MR)

came into use; the electrical resistance of the head changed according to the

strength of the magnetism from the platter. Later development made use of

spintronics; in these heads, the magnetoresistive effect was much greater that in

earlier types, and was dubbed "giant" magnetoresistance (GMR). This refers to the

degree of effect, not the physical size, of the head — the heads themselves are

extremely tiny, and are too small to be seen without a microscope. GMR read heads

are now commonplace.[citation needed]

HD heads are kept from contacting the platter surface by the air that is extremely

close to the platter; that air moves at, or close to, the platter speed. [citation needed] The

record and playback head are mounted on a block called a slider, and the surface

next to the platter is shaped to keep it just barely out of contact. It's a type of air

bearing.

The magnetic surface of each platter is conceptually divided into many small sub-

micrometre-sized magnetic regions, each of which is used to encode a single binary

unit of information. In today's HDDs, each of these magnetic regions is composed of

a few hundred magnetic grains. Each magnetic region forms a magnetic dipole

which generates a highly localized magnetic field nearby. The write head

magnetizes a region by generating a strong local magnetic field. Early HDDs used

an electromagnet both to generate this field and to read the data by using

electromagnetic induction. Later versions of inductive heads included metal in Gap

(MIG) heads and thin film heads. In today's heads, the read and write elements are

separate, but in close proximity, on the head portion of an actuator arm. The read

element is typically magneto-resistive while the write element is typically thin-film

inductive.[5]

In modern drives, the small size of the magnetic regions creates the danger that

their magnetic state might be lost because of thermal effects. To counter this, the

platters are coated with two parallel magnetic layers, separated by a 3-atom-thick

layer of the non-magnetic element ruthenium, and the two layers are magnetized in

opposite orientation, thus reinforcing each other.[6] Another technology used to

overcome thermal effects to allow greater recording densities is perpendicular

recording, first shipped in 2005[7], as of 2007 the technology was used in many

HDDs[8][9][10].

See File System for how operating systems access data on HDDs and other storage

devices.

[edit] Architecture

A hard disk drive with the platters and spindle motor hub removed showing the copper

colored motor coils surrounding a bearing at the center of the spindle motor.

The motor has an external rotor; the stator windings are copper-colored. The spindle

bearing is in the center. To the left of center is the actuator with a read-write head

under the tip of its very end (near center); the orange stripe along the side of the

arm, a thin printed-circuit cable, connects the read-write head to the hub of the

actuator. The flexible, somewhat 'U'-shaped, ribbon cable barely visible below and to

the left of the actuator arm is the flexible section, one end on the hub, that continues

the connection from the head to the controller board on the opposite side.

The head support arm is very light, but also rigid; in modern drives, acceleration at

the head reaches 250 g's.

The silver-colored structure at the upper left is the top plate of the permanent-

magnet and moving coil "motor" that swings the heads to the desired position.

Beneath this plate is the moving coil, attached to the actuator hub, and beneath that

is a thin neodymium-iron-boron (NIB) high-flux magnet. That magnet is mounted on

the bottom plate of the "motor".

The coil, itself, is shaped rather like an arrowhead, and made of doubly-coated

copper magnet wire. The inner layer is insulation, and the outer is thermoplastic,

which bonds the coil together after it's wound on a form, making it self-supporting.

Much of the coil, sides of the arrowhead, which points to the actuator bearing center,

interacts with the magnetic field to develop a tangential force to rotate the actuator.

Considering that current flows (at a given time) radially outward along one side of

the arrowhead, and radially inward on the other, the surface of the magnet is half N

pole, half S pole; the dividing line is midway, and radial.

[edit] Capacity and access speed

PC hard disk drive capacity (in GB). The vertical axis is logarithmic, so the fit line

corresponds to exponential growth.

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy

disk drive. Consequently, hard disk drives can store much more data than floppy

disk drives and can access and transmit it faster. As of January 2008:

A typical desktop HDD, might store between 120 and 300 GB of data (based on US

market data[11]), rotate at 7,200 revolutions per minute (RPM) and have a media

transfer rate of 1 Gbit/s or higher. (1 GB = 109 B; 1 Gbit/s = 109 bit/s)

The highest capacity HDDs are 1 TB.

The fastest “enterprise” HDDs spin at 10,000 or 15,000 rpm, and can achieve

sequential media transfer speeds above 1.6 Gbit/s.[12] and a sustained transfer rate

up to 125MBytes/second.[13] Drives running at 10,000 or 15,000 rpm use smaller

platters because of air drag and therefore generally have lower capacity than the

highest capacity desktop drives.

Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and

enterprise counterparts, tend to be slower and have less capacity. A typical mobile

HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium.

Because of the smaller disks, mobile HDDs generally have lower capacity than the

highest capacity desktop drives.

The exponential increases in disk space and data access speeds of HDDs have

enabled the commercial viability of consumer products that require large storage

capacities, such as digital video recorders and digital audio players.[14] In addition,

the availability of vast amounts of cheap storage has made viable a variety of web-

based services with extraordinary capacity requirements, such as free-of-charge

web search, web archiving and video sharing (Google, Yahoo!, YouTube, etc.).

The main way to decrease access time is to increase rotational speed, while the

main way to increase throughput and storage capacity is to increase areal density. A

vice president of Seagate Technology projects a future growth in disk density of 40%

per year.[15] Access times have not kept up with throughput increases, which

themselves have not kept up with growth in storage capacity.

The first 3.5" HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000.

It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable

space.[16] Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which

has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.[17][18]

Form factor Width Largest capacity Platters (Max)

5.25" FH 146 mm 47 GB[19] (1998) 14

5.25" HH 146 mm 19.3 GB[20] (1998) 4[21]

3.5" 102 mm 1 TB[16] (2007) 5

2.5" 69.9 mm 500 GB[22] (2008) 3

1.8" (PCMCIA) 54 mm 160 GB[23] (2007)

1.8" (ATA-7 LIF) 53.8 mm

1.3" 36.4 mm 40 GB[24] (2008) 1

[edit] Capacity measurements

It has been suggested that Disk Overhead be merged into this article or section. (Discuss)

A disassembled and labeled 1997 hard drive.

Capacity of a hard disk drive is usually quoted in gigabytes and terabytes. Older

HDDs quoted their smaller capacities in megabytes, the first drives for PCs being

just 5 or 10 MB.

The capacity of an HDD can be calculated by multiplying the number of cylinders by

the number of heads by the number of sectors by the number of bytes/sector (most

commonly 512). Drives with ATA interface bigger and more than eight gigabytes

behave as if they were structured into 16383 cylinders, 16 heads, and 63 sectors, for

compatibility with older operating systems. Unlike in the 1980s, the cylinder, head,

sector (C/H/S) counts reported to the CPU by a modern ATA drive are no longer

actual physical parameters since the reported numbers are constrained by historic

operating-system interfaces and with zone bit recording the actual number of sectors

varies by zone. Disks with SCSI interface address each sector with a unique integer

number; the operating system remains ignorant of their head or cylinder count.

The old C/H/S scheme has been replaced by logical block addressing. In some

cases, to try to "force-fit" the C/H/S scheme to large-capacity drives, the number of

heads was given as 64, although no drive has anywhere near 32 platters.

Hard disk drive manufacturers specify disk capacity using the SI prefixes mega-,

giga- and tera-, and their abbreviations M, G and T. Byte is typically abbreviated B.

Most operating-system tools report capacity using the same abbreviations but

actually use binary prefixes. For instance, the prefix mega-, which normally means

106 (1,000,000), in the context of data storage can mean 220 (1,048,576), which is

nearly 5% more. Similar usage has been applied to prefixes of greater magnitude.

This results in a discrepancy between the disk manufacturer's stated capacity and

the apparent capacity of the drive when examined through most operating-system

tools. The difference becomes even more noticeable (7%) for a gigabyte. For

example, Microsoft Windows reports disk capacity both in decimal-based units to 12

or more significant digits and with binary-based units to three significant digits. Thus

a disk specified by a disk manufacturer as a 30 GB disk might have its capacity

reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB". The disk

manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however,

because the utilities provided by Windows, Mac and some Linux distributions define

a gigabyte as 1,073,741,824 bytes (230 bytes, often referred to as a gibibyte, or GiB),

the operating system reports capacity of the disk drive as (only) 28.0 GB.

[edit] Form factors

5¼" full height 110 MB HDD,

2½" 8.5 mm 6495 MB HDD,

US/UK pennies for comparison

Six hard drives with 8", 5.25", 3.5", 2.5", 1.8", and 1" disks, partially disassembled to show

platters and read-write heads, with a ruler showing inches.

The earliest “form factor” hard disk drives inherited their dimensions from floppy-disk

drives (FDDs), so that either could be mounted in chassis slots, and thus the HDD

form factors became colloquially named after the corresponding FDD types. "Form

factor" compatibility continued after the 3½ in size even though floppy disk drives

with new smaller dimensions ceased to be offered.

8 inch: (9.5 in x 4.624 in x 14.25 in = 241.3 mm x 117.5 mm x 362 mm)

In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD,

having the same dimensions and a compatible interface to the 8" FDD. Both "full

height" and "half height" (2.313 in) versions were available.

5.25 inch: (5.75 in x 1.63 in x 8 in = 146.1 mm x 41.4 mm x 203 mm)

This smaller form factor, first used in an HDD by Seagate in 1980, was the same

size as full height 5¼-inch diameter FDD, i.e., 3.25 inches high. This is twice as high

as commonly used today; i.e., 1.63 in = 41.4 mm (“half height”). Most desktop

models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼"

dimension, but it fell out of fashion for HDDs. The Quantum Bigfoot HDD was the

last to use it in the late 1990s, with “low-profile” (~25 mm) and “ultra-low-profile” (~20

mm) high versions.

3.5 inch (4 in x 1 in x 5.75 in = 101.6 mm x 25.4 mm x 146 mm)

This smaller form factor, first used in an HDD by Rodime in 1984, was the same size

as the "half height" 3½ FDD, i.e., 1.63 inches high. Today has been largely

superseded by 1-inch high “slimline” or “low-profile” versions of this form factor

which is used by most desktop HDDs.

2.5 inch: (2.75 in x 0.374 in x 3.945 in = 69.85 mm x 9.5–15mm mm x 100 mm)

This smaller form factor was introduced by PrairieTek in 1988; there is no

corresponding FDD. It is widely used today for hard-disk drives in mobile devices

(laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class

drives. Today, the dominant height of this form factor is 9.5 mm for laptop drives, but

high capacity drives have a height of 12.5 mm. Enterprise-class drives can have a

height up to 15 mm.[25]

1.8inch: (54 mm × 8 mm × 71 mm)

This form factor, originally introduced by Integral Peripherals in 1993, has

evolved into the ATA-7 LIF with dimensions as stated. It is increasingly used in

digital audio players and subnotebooks. An original variant exists for 2–5 GB

sized HDDs that fit directly into a PC card expansion slot. These became popular

for their use in iPods and other HDD based MP3 players. 1 inch: (42.8 mm × 5

mm × 36.4 mm)

This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF

Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product

literature.[26]

0.85 inch: (24 mm × 5 mm × 32 mm)

Toshiba announced this form factor in January 2004[27] for use in mobile phones and

similar applications, including SD/MMC slot compatible HDDs optimized for video

storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB

(MK8003MTD) version[4] and holds the Guinness World Record for the smallest

harddisk drive.[28]

Major manufacturers discontinued the development of new products for the 1-inch

(=1.3-inch) and 0.85-inch form factors in 2007, due to falling prices of flash

memory[29], although Samsung introduced in 2008 with the SpinPoint A1 another 1.3-

inch drive.

The inch-based nickname of all these form factors usually do not indicate any actual

product dimension (which are specified in millimeters for more recent form factors),

but just roughly indicate a size relative to disk diameters, in the interest of historic

continuity.

[edit] Other characteristics

Transfer rate As of 2008, the data transfer rate at the inner zone ranges from 44.2

MB/s to 74.5 MB/s, while the transfer rate at the outer zone ranges from 74.0 MB/s

to 111.4 MB/s.[citation needed] In contrast, the first PC drives could manage only around

40 KiB/s.

Random access time (seek time) currently ranges from just under 5 ms for high-end

server drives, to 15 ms for miniature drives, with the most common desktop type

typically being around 9 ms.[citation needed] There has not been any significant

improvement in this speed for some years. Some early PC drives used a worm-gear

to move the heads, and as a result had access times as slow as 80 - 120 ms, but

this was quickly improved by voice-coil type actuation in the late 1980s, seeing

access times reduce to around 20 ms.

Power consumption has become increasingly important, not just in mobile devices

such as laptops but also in server and desktop markets. Increasing data center

machine density has led to problems delivering sufficient power to devices, and

getting rid of the waste heat subsequently produced, as well as environmental and

electrical cost concerns (see green computing). Similar issues exist for large

companies with thousands of desktop PCs. Smaller form factor drives often use less

power than larger drives. One interesting development in this area is actively

controlling the seek speed so that the head arrives at its destination only just in time

to read the sector, rather than arriving as quickly as possible and then having to wait

for the sector to come around (i.e. the rotational latency).

Audible noise (measured in dBA) is significant for certain applications, such as

PVRs digital audio recording and quiet computers. Low noise disks typically use fluid

bearings, slower rotational speeds (usually 5,400rpm) and reduce the seek speed

under load (AAM) to reduce audible clicks and crunching sounds. Drives in smaller

form factors (e.g. 2.5 inch) are often quieter than larger drives.

Shock resistance is especially important for mobile devices. Some laptops now

include a motion sensor that parks the disk heads if the machine is dropped,

hopefully before impact, to offer the greatest possible chance of survival in such an

event.

[edit] Access and interfacesThis section does not cite any references or sources. (May 2008)Please help improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed.

Hard disk drives are accessed over one of a number of bus types, including parallel

ATA (PATA, also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached

SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard

disk drives to buses that they cannot communicate with natively, such as IEEE 1394

and USB.

Back in the days of the ST-506 interface, the data encoding scheme was also

important. The first ST-506 disks used Modified Frequency Modulation (MFM)

encoding, and transferred data at a rate of 5 megabits per second. Later on,

controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate by

50%, to 7.5 megabits per second; this also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at

the lower MFM data rate, while other models (usually more expensive versions of

the same basic disk drive) were specified to run at the higher RLL data rate. In some

cases, a disk drive had sufficient margin to allow the MFM specified model to run at

the faster RLL data rate; however, this was often unreliable and was not

recommended. (An RLL-certified disk drive could run on a MFM controller, but with

1/3 less data capacity and speed.)

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI

disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was

usually negotiated automatically by the disk drive and controller; most of the time,

however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15

or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives

typically also had jumpers to set the number of sectors per track and (in some

cases) sector size.

SCSI originally had just one speed, 5 MHz (for a maximum data rate of five

megabytes per second), but later this was increased dramatically. The SCSI bus

speed had no bearing on the disk's internal speed because of buffering between the

SCSI bus and the disk drive's internal data bus; however, many early disk drives had

very small buffers, and thus had to be reformatted to a different interleave (just like

ST-506 disks) when used on slow computers, such as early IBM PC compatibles

and early Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their

controller design, but many early models were incompatible with each other and

couldn't run in a master/slave setup (two disks on the same cable). This was mostly

remedied by the mid-1990s, when ATA's specification was standardised and the

details began to be cleaned up, but still causes problems occasionally (especially

with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA

devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its

own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally

ATA or SCSI disks with ports on the back allowing very simple and effective

expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in

order to continue adding peripherals without requiring additional ports on the

computer itself.

[edit] Disk interface families used in personal computers

Notable families of disk interfaces include:

Historical bit serial interfaces — connected to a hard disk drive controller with three

cables, one for data, one for control and one for power. The HDD controller provided

significant functions such as serial to parallel conversion, data separation and track

formatting, and required matching to the drive in order to assure reliability.

ST506 used MFM (Modified Frequency Modulation) for the data encoding method.

ST412 was available in either MFM or RLL (Run Length Limited) variants.

Enhanced Small Disk Interface (ESDI) was an interface developed by Maxtor to

allow faster communication between the PC and the disk than MFM or RLL.

Modern bit serial interfaces — connect to a host bus adapter (today typically

integrated into the "south bridge") with two cables, one for data/control and one for

power.

Fibre Channel (FC), is a successor to parallel SCSI interface on enterprise market. It

is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL)

connection topology is used. FC has much broader usage than mere disk interfaces,

it is the cornerstone of storage area networks (SANs). Recently other protocols for

this field, like iSCSI and ATA over Ethernet have been developed as well.

Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not

fibre optics. The latter are traditionally reserved for larger devices, such as servers

or disk array controllers.

Serial ATA (SATA). The SATA data cable has one data pair for differential

transmission of data to the device, and one pair for differential receiving from the

device, just like EIA-422. That requires that data be transmitted serially. The same

differential signaling system is used in RS485, LocalTalk, USB, Firewire, and

differential SCSI.

Serial Attached SCSI (SAS). The SAS is a new generation serial communication

protocol for devices designed to allow for much higher speed data transfers and is

compatible with SATA. SAS uses serial communication instead of the parallel

method found in traditional SCSI devices but still uses SCSI commands.

Word serial interfaces — connect to a host bus adapter (today typically integrated

into the "south bridge") with two cables, one for data/control and one for power. The

earliest versions of these interfaces typically had a 16 bit parallel data transfer

to/from the drive and there are 8 and 32 bit variants. Modern versions have serial

data transfer. The word nature of data transfer makes the design of a host bus

adapter significantly simpler than that of the precursor HDD controller.

Integrated Drive Electronics (IDE), later renamed to ATA, and then later to PATA

("parallel ATA", to distinguish it from the new Serial ATA). The original name

reflected the innovative integration of HDD controller with HDD itself, which was not

found in earlier disks. Moving the HDD controller from the interface card to the disk

drive helped to standardize interfaces, including reducing the cost and complexity.

The 40 pin IDE/ATA connection of PATA transfers 16 bits of data at a time on the

data cable. The data cable was originally 40 conductor, but later higher speed

requirements for data transfer to and from the hard drive led to an "ultra DMA"

mode, known as UDMA, which required an 80 conductor variant of the same cable;

the other conductors provided the grounding necessary for enhanced high-speed

signal quality. The interface for 80 pin only has 39 pins, the missing pin acting as a

key to prevent incorrect insertion of the connector to an incompatible socket, a

common cause of disk and controller damage.

EIDE was an unofficial update (by Western Digital) to the original IDE standard, with

the key improvement being the use of direct memory access (DMA) to transfer data

between the disk and the computer without the involvement of the CPU, an

improvement later adopted by the official ATA standards. By directly transferring

data between memory and disk, DMA does not require the CPU/program/operating

system to leave other tasks idle while the data transfer occurs.

Small Computer System Interface (SCSI), originally named SASI for Shugart

Associates System Interface, was an early competitor of ESDI. SCSI disks were

standard on servers, workstations, and Apple Macintosh computers through the mid-

90s, by which time most models had been transitioned to IDE (and later, SATA)

family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk

technology, though the highest-performance disks are still available in SCSI and

Fibre Channel only. The length limitations of the data cable allows for external SCSI

devices. Originally SCSI data cables used single ended data transmission, but

server class SCSI could use differential transmission, either low voltage differential

(LVD) or high voltage differential (HVD).

Acronym or abbreviation

Meaning Description

SASIShugart Associates System Interface

Historical predecessor to SCSI.

SCSISmall Computer System Interface

Bus oriented that handles concurrent operations.

SAS Serial Attached SCSIImprovement of SCSI, uses serial communication instead of parallel.

ST-506 Historical Seagate interface.

ST-412Historical Seagate interface (minor improvement over ST-506).

ESDIEnhanced Small Disk Interface

Historical; backwards compatible with ST-412/506, but faster and more integrated.

ATAAdvanced Technology Attachment

Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.

SATA Serial ATAImprovement of ATA, uses serial communication instead of parallel.

[edit] Integrity

An IBM HDD head resting on a disk platter. Since the drive is not in operation, the head is

simply pressed against the disk by the suspension.

Close-up of a hard disk head resting on a disk platter, and its suspension. A reflection of the

head and suspension are visible beneath on the mirror-like disk.

Due to the extremely close spacing between the heads and the disk surface, any

contamination of the read-write heads or platters can lead to a head crash — a

failure of the disk in which the head scrapes across the platter surface, often

grinding away the thin magnetic film and causing data loss. Head crashes can be

caused by electronic failure, a sudden power failure, physical shock, wear and tear,

corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the enclosure to support the

heads at their proper flying height while the disk rotates. An HDD requires a certain

range of air pressures in order to operate properly. The connection to the external

environment and pressure occurs through a small hole in the enclosure (about 0.5

mm in diameter), usually with a carbon filter on the inside (the breather filter, see

below). If the air pressure is too low, then there is not enough lift for the flying head,

so the head gets too close to the disk, and there is a risk of head crashes and data

loss. Specially manufactured sealed and pressurized disks are needed for reliable

high-altitude operation, above about 3,000 m (10,000 feet). Note that modern

commercial aircraft have a pressurized cabin, whose pressure altitude does not

normally exceed 2,600 m(8,500 feet) - thus, ordinary hard drives can safely be used

in flight. Modern disks include temperature sensors and adjust their operation to the

operating environment. Breather holes can be seen on all disks — they usually have

a sticker next to them, warning the user not to cover the holes. The air inside the

operating disk is constantly moving too, being swept in motion by friction with the

spinning platters. This air passes through an internal recirculation (or "recirc") filter to

remove any leftover contaminants from manufacture, any particles or chemicals that

may have somehow entered the enclosure, and any particles or outgassing

generated internally in normal operation. Very high humidity for extended periods

can corrode the heads and platters.

For giant magnetoresistive (GMR) heads in particular, a minor head crash from

contamination (that does not remove the magnetic surface of the disk) still results in

the head temporarily overheating, due to friction with the disk surface, and can

render the data unreadable for a short period until the head temperature stabilizes

(so called "thermal asperity," a problem which can partially be dealt with by proper

electronic filtering of the read signal).

The hard disk's electronics control the movement of the actuator and the rotation of

the disk, and perform reads and writes on demand from the disk controller. Modern

disk firmware is capable of scheduling reads and writes efficiently on the platter

surfaces and remapping sectors of the media which have failed.

[edit] Landing zones and load/unload technology

Microphotograph of a hard disk head. The size of the front face (which is the "trailing face"

of the slider) is about 0.3 mm × 1.0 mm. The (not visible) bottom face of the slider is about

1.0 mm × 1.25 mm (so called "nano" size) and faces the platter. One functional part of the

head is the round, orange structure in the middle - the lithographically defined copper coil of

the write transducer. Also note the electric connections by wires bonded to gold-plated

pads.

Most HDDs prevent power interruptions from shutting the drive down with its heads

landing in the data zone by either moving the heads to a landing zone or unloading

(i.e., load/unload) the heads.

A landing zone is an area of the platter usually near its inner diameter (ID), where

no data is stored. This area is called the Contact Start/Stop (CSS) zone. Disks are

designed such that either a spring or, more recently, rotational inertia in the platters

is used to park the heads in the case of unexpected power loss. In this case, the

spindle motor temporarily acts as a generator, providing power to the actuator.

Spring tension from the head mounting constantly pushes the heads towards the

platter. While the disk is spinning, the heads are supported by an air bearing and

experience no physical contact or wear. In CSS drives the sliders carrying the head

sensors (often also just called heads) are designed to survive a number of landings

and takeoffs from the media surface, though wear and tear on these microscopic

components eventually takes its toll. Most manufacturers design the sliders to

survive 50,000 contact cycles before the chance of damage on startup rises above

50%. However, the decay rate is not linear: when a disk is younger and has had

fewer start-stop cycles, it has a better chance of surviving the next startup than an

older, higher-mileage disk (as the head literally drags along the disk's surface until

the air bearing is established). For example, the Seagate Barracuda 7200.10 series

of desktop hard disks are rated to 50,000 start-stop cycles, in other words no failures

attributed to the head-platter interface were seen before at least 50,000 start-stop

cycles during testing.[30]

Around 1995 IBM pioneered a technology where a landing zone on the disk is made

by a precision laser process (Laser Zone Texture = LZT) producing an array of

smooth nanometer-scale "bumps" in a landing zone, thus vastly improving stiction

and wear performance. This technology is still largely in use today (2007),

predominantly in desktop and enterprise (3.5 inch) drives. In general, CSS

technology can be prone to increased stiction (the tendency for the heads to stick to

the platter surface), e.g. as a consequence of increased humidity. Excessive stiction

can cause physical damage to the platter and slider or spindle motor.

Load/Unload technology relies on the heads being lifted off the platters into a safe

location, thus eliminating the risks of wear and stiction altogether. The first HDD

RAMAC and most early disk drives used complex mechanisms to load and unload

the heads. Modern HDDs use ramp loading, first introduced by Memorex in 1967 [31],

to load/unload onto plastic "ramps" near the outer disk edge.

All HDDs today still use one of these two technologies. Each has a list of

advantages and drawbacks in terms of loss of storage area on the disk, relative

difficulty of mechanical tolerance control, cost of implementation, etc.

Addressing shock robustness, IBM also created a technology for their ThinkPad line

of laptop computers called the Active Protection System. When a sudden, sharp

movement is detected by the built-in accelerometer in the Thinkpad, internal hard

disk heads automatically unload themselves to reduce the risk of any potential data

loss or scratch defects. Apple later also utilized this technology in their PowerBook,

iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor.

Toshiba has released similar technology in their laptops.[32]

[edit] Disk failures and their metricsWikibooks has a book on the topic of

Minimizing hard disk drive failure and data loss

Most major hard disk and motherboard vendors now support self-monitoring,

analysis and reporting technology (S.M.A.R.T.), which attempts to alert users to

impending failures.

However, not all failures are predictable. Normal use eventually can lead to a

breakdown in the inherently fragile device, which makes it essential for the user to

periodically back up the data onto a separate storage device. Failure to do so will

lead to the loss of data. While it may sometimes be possible to recover lost

information, it is normally an extremely costly procedure, and it is not possible to

guarantee success. A 2007 study published by Google suggested very little

correlation between failure rates and either high temperature or activity level.[33]

While several S.M.A.R.T. parameters have an impact on failure probability, a large

fraction of failed drives do not produce predictive S.M.A.R.T. parameters.[33]

S.M.A.R.T. parameters alone may not be useful for predicting individual drive

failures.[33]

A common misconception is that a colder hard drive will last longer than a hotter

hard drive. The Google study showed the reverse -- "lower temperatures are

associated with higher failure rates". Hard drives with S.M.A.R.T.-reported average

temperatures below 27 C had failure rates worse than hard drives with the highest

reported average temperature of 50 C, failure rates at least twice as high as the

optimum S.M.A.R.T.-reported temperature range of 36 C to 47 C.[33]

SCSI, SAS and FC drives are typically more expensive and are traditionally used in

servers and disk arrays, whereas inexpensive ATA and SATA drives evolved in the

home computer market and were perceived to be less reliable. This distinction is

now becoming blurred.

The mean time between failures (MTBF) of SATA drives is usually about 600,000

hours (some drives such as Western Digital Raptor have rated 1.2 million hours

MTBF), while SCSI drives are rated for upwards of 1.5 million hours. [citation needed]

However, independent research indicates that MTBF is not a reliable estimate of a

drive's longevity.[34] MTBF is conducted in laboratory environments in test chambers

and is an important metric to determine the quality of a disk drive before it enters

high volume production. Once the drive product is in production, the more valid[citation

needed] metric is annualized failure rate (AFR). AFR is the percentage of real-world

drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high [citation needed]

reliability.

Enterprise SATA drives designed and produced for enterprise markets, unlike

standard SATA drives, have reliability comparable to other enterprise class drives.[citation needed]

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise

SATA and FC) experience between 0.70%-0.78% annual failure rates from the total

installed drives.[citation needed]

[edit] Manufacturers

A Western Digital 3.5 inch 250 GB SATA HDD. This drive has an optional 4-pin Molex

power connector typically used for the older ATA standard, and SATA typically uses its own,

proprietary power connector.

See also List of defunct hard disk manufacturers

The technological resources and know-how required for modern drive development

and production mean that as of 2007, over 98% of the world's HDDs are

manufactured by just a handful of large firms: Seagate (which now owns Maxtor),

Western Digital, Samsung, and Hitachi (which owns the former disk manufacturing

division of IBM). Fujitsu continues to make mobile- and server-class disks but exited

the desktop-class market in 2001. Toshiba is a major manufacturer of 2.5-inch and

1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.

Dozens of former HDD manufacturers have gone out of business, merged, or closed

their HDD divisions; as capacities and demand for products increased, profits

became hard to find, and the market underwent significant consolidation in the late

1980s and late 1990s. The first notable casualty of the business in the PC era was

Computer Memories Inc. or CMI; after an incident with faulty 20 MB AT disks in

1985,[35] CMI's reputation never recovered, and they exited the HDD business in

1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it

was found that they had engaged in accounting fraud and inflated sales numbers for

several years. Many other smaller companies (like Kalok, Microscience, LaPine,

Areal, Priam and PrairieTek) also did not survive the shakeout, and had disappeared

by 1993; Micropolis was able to hold on until 1997, and JTS, a relative latecomer to

the scene, lasted only a few years and was gone by 1999, after attempting to

manufacture HDDs in India. Their claim to fame was creating a new 3" form factor

drive for use in laptops. Quantum and Integral also invested in the 3" form factor; but

eventually gave up as this form factor failed to catch on.[citation needed] Rodime was also

an important manufacturer during the 1980s, but stopped making disks in the early

1990s amid the shakeout and now concentrates on technology licensing; they hold a

number of patents related to 3.5-inch form factor HDDs.

This list is incomplete; you can help by expanding it.

1988: Tandon Corporation sold its disk manufacturing division to Western Digital

(WDC), which was then a well-known controller designer.[36]

1989: Seagate Technology bought Control Data's high-end disk business, as part of

CDC's exit from hardware manufacturing.

1990: Maxtor buys MiniScribe out of bankruptcy, making it the core of its low-end

disk division.

1994: Quantum bought DEC's storage division, giving it a high-end disk range to go

with its more consumer-oriented ProDrive range, as well as the DLT tape drive

range.

1995: Conner Peripherals, which was founded by one of Seagate Technology's co-

founders along with personnel from MiniScribe, announces a merger with Seagate,

which was completed in early 1996.

1996: JTS merges with Atari, allowing JTS to bring its disk range into production.

Atari was sold to Hasbro in 1998, while JTS itself went bankrupt in 1999.

2000: Quantum sells its disk division to Maxtor to concentrate on tape drives and

backup equipment.

2003: Following the controversy over mass failures of its Deskstar 75GXP range,

HDD pioneer IBM sold the majority of its disk division to Hitachi, who renamed it

Hitachi Global Storage Technologies (HGST).

December 21, 2005: Seagate and Maxtor announced an agreement under which

Seagate would acquire Maxtor in an all stock transaction valued at $1.9 billion. The

acquisition was approved by the appropriate regulatory bodies, and closed on May

19, 2006.

2006: Western Digital releases the Western Digital Raptor X.[37]

2007 April: Hitachi releases the 1 TB Hitachi Deskstar 7K1000 (1TB = 1 trillion

bytes, roughly 931.5 GiB).[38][39][40]

July: Western Digital (WDC) acquires Komag U.S.A, a thin-film media manufacturer,

for USD 1 Billion.[41]

September: Hitachi releases 2.5-inch 320 GB hard disk.

2008 January: Hitachi releases 2.5-inch 500 GB hard disk and it was demonstrated

by Asus with a laptop at 1 TB storage in CES event, Las Vegas

April 21: Western Digital releases the new VelociRaptor hard drive.[42]