Strictly speaking, "drive" refers to an entire unit containing hard disk, read/write head assembly, driver electronics, and motor while "hard disk" (sometimes "platter") refers to the storage medium itself.
Hard disks were originally developed for use with computers. In the 21st century, applications for hard disks have expanded beyond computers to include video recorders, audio players, digital organizers, and digital cameras. In 2005 the first cellular telephones to include hard disks were introduced by Samsung and Nokia. The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID, hardware such as network attached storage (NAS) devices, and systems such as storage area networks (SANs) for efficient access to large volumes of data.
Hard disks record information by magnetizing a magnetic material in a pattern that represents the data. They read the data back by detecting the magnetization of the material. A typical hard disk 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 glass or aluminum, 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.
The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that fly very close 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 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.
A cross section of the magnetic surface in action. In this case the binary data encoded using frequency modulation.The magnetic surface of each platter is 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 hard disks each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localised magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early hard disks used the same inductor that was used to read the data as an electromagnet to create this field. Later, metal in Gap (MIG) heads were used, and today thin film heads are common. With these later technologies, the read and write head are separate mechanisms, but are on the same actuator arm.
Hard disks have a mostly sealed enclosure that protects the disk internals from dust, condensation, and other sources of contamination. The hard disk's read-write heads fly on an air bearing which is a cushion of air only nanometers above the disk surface. The disk surface and the disk's internal environment must therefore be kept immaculate to prevent damage from fingerprints, hair, dust, smoke particles and such, given the sub-microscopic gap between the heads and disk.
Using rigid platters and sealing the unit allows much tighter tolerances than in a floppy disk. Consequently, hard disks can store much more data than floppy disk and access and transmit it faster. In 2006, a typical workstation hard disk might store between 80 GB and 1Tb of data, rotate at 7,200 to 10,000 revolutions per minute (RPM), and have a sequential media transfer rate of over 50 MB/s. The fastest workstation and server hard disks spin at 15,000 RPM, and can achieve sequential media transfer speeds up to and beyond 80 MB/s. Laptop hard disks, which are physically smaller than their desktop counterparts, tend to be slower and have less capacity. Most spin at only 4,200 RPM or 5,400 RPM, whereas the newest top models spin at 7,200 RPM.
The capacity of hard disks has grown dramatically over time. The first commercial disk, the IBM RAMAC introduced in 1956, stored 5 million characters (about 5 megabytes) on fifty 24-inch diameter disks. (See early IBM disk storage.) With early personal computers in the 1980s, a disk with a 20 megabyte capacity was considered large. In the latter half of the 1990s, hard disks with capacities of 1 gigabyte and greater became available. As of 2006, the "smallest" desktop hard disk still in production has a capacity of 20 gigabytes, while the largest-capacity internal disks are a 3/4 terabyte (750 gigabytes), with external disks at or exceeding one terabyte by using multiple internal disks. These new internal disks increased their storage capacities with perpendicular recording.
This has enabled the commercial viability of consumer products that require large storage capacities, such as the Apple iPod digital music player, the TiVo personal video recorder, and web-based email programs. This is also gradually but significantly altering how programmers think; in many programming tasks there is a time-space tradeoff, so as space becomes cheaper and cheaper relative to CPU cycles the appropriate choice about time versus space changes. For instance in database work it is now common practice to store precomputed views, transitive closures, and the like on disk in order to speed up queries; 20 years ago such profligate use of disk space would have been impractical.
A vice president of Seagate projects a future growth in disk density of 40% per year. Access times have not kept up with throughput increases, which themselves haven't kept up with growth in storage capacity. The main way to increase either is to increase the number of read-write heads in a hard disk. Since flying heads are the most expensive component of hard disks, increasing their number per hard disk wouldn't help the situation. Currently, the most promising way to reduce access times and increase throughput are to replace rotating disks with nonvolatile random access memory (NVRAM) or, possibly, holographic technology.
Hard disk manufacturers typically specify disk capacity using the SI definition of the prefixes "mega" and "giga." This is largely for historical reasons. Disks with multi-million byte capacity have been used since 1956, long before there were standard binary prefixes. (The IEC only standardized binary prefixes in 1999.) Many practitioners early on in the computer and semiconductor industries used the prefix kilo to describe 210 (1024) bits, bytes or words because 1024 is "close enough" to 1000. Similar usage has been applied to the prefixes "mega," "giga," "tera," and even "peta." Often this non-SI conforming usage is noted by a qualifier such as "1 kB = 1,024 bytes" but the qualifier is sometimes omitted, particularly in marketing literature.
Operating systems, such as Microsoft Windows, frequently report capacity using the binary interpretation of the prefixes, which results in a discrepancy between the disk manufacturer's stated capacity and what the system reports. The difference becomes much more noticeable in the multi-gigabyte range. For example, Microsoft's Windows 2000 reports disk capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 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. However utilities provided by Windows define a gigabyte as 230, or 1073741824, bytes, so the reported capacity of the disk will be closer to 28.0 GB. For this reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes (e.g. KiB, MiB, GiB) since their definitions are unambiguous.
Some people mistakenly attribute the discrepancy in reported and specified capacities to reserved space used for file system and partition accounting information. However, for large (several GiB) filesystems, this data rarely occupies more than a few MiB, and therefore cannot possibly account for the apparent "loss" of tens of GBs.
The capacity of a hard disk 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).
IBM 62PC "Piccolo" HDD, circa 1979 - an early 8" diskFor many years, hard disks were large, cumbersome devices, more suited to use in the protected environment of a data center or large office than in a harsh industrial environment (due to their delicacy), or small office or home (due to their size and power consumption). Before the early 1980s, most hard disks had 8-inch (20 cm) or 14-inch (35 cm) platters, required an equipment rack or a large amount of floor space (especially the large removable-media disks, which were often referred to as "washing machines"), and in many cases needed high-current or even three-phase power hookups due to the large motors they used. Because of this, hard disks were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25-inch hard disk, with a capacity of 5 megabytes. In fact, in its factory configuration, the original IBM PC (IBM 5150) was not equipped with a hard disk.
Most microcomputer hard disks in the early 1980s were not sold under their manufacturer's names, but by OEMs as part of larger peripherals (such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal hard disk, however, and this started a trend toward buying "bare" disks (often by mail order) and installing them directly into a system. Hard disk makers started marketing to end users as well as OEMs, and by the mid-1990s, hard disks had become available on retail store shelves.
While internal disks became the system of choice on PCs, external hard disks remained popular for much longer on the Apple Macintosh and other platforms. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy. External SCSI disks were also popular with older microcomputers such as the Apple II series, and were also used extensively in servers, a usage which is still popular today. The appearance in the late 1990s of high-speed external interfaces such as USB and FireWire has made external disk systems popular among PC users once again, especially for users who move large amounts of data between two or more locations, and most hard disk makers now make their disks available in external cases.
Hard disk characteristics
5.25" MFM 110 MB hard disk (2.5" IDE 6495 MB hard disk, US & UK pennies for comparison)Capacity, usually quoted in gigabytes. (older hard disks used to quote their smaller capacities in megabytes)
Physical size, usually quoted in inches:
Almost all hard disks today are of either the 3.5" or 2.5" varieties, used in desktops and laptops, respectively. 2.5" disks are usually slower and have less capacity but use less power and are more tolerant of movement. An increasingly common size is the 1.8" disks used in portable MP3 players and subnotebooks, which have very low power consumption and are highly shock-resistant. Additionally, there is the 1" form factor designed to fit the dimensions of CF Type II, which is also usually used as storage for portable devices including digital cameras. 1" was a de facto form factor led by IBM's Microdrive, but is now generically called 1" due to other manufacturers producing similar products. There is also a 0.85" form factor produced by Toshiba for use in mobile phones and similar applications. The size designations can be slightly confusing, for example a 3.5" disk has a case that is 4" wide. Furthermore, server-class hard disks also come in both 3.5" and 2.5" form factors.
Reliability, usually given in terms of Mean Time Between Failures (MTBF):
SATA 1.0 disks support speeds up to 10,000 rpm and MTBF levels up to 1 million hours under an eight-hour, low-duty cycle. Fibre Channel (FC) disks support up to 15,000 rpm and an MTBF of 1.4 million hours under a 24-hour duty cycle.
Number of I/O operations per second:
Modern disks can perform around 50 random access or 100 Sequential access operations per second.
Power consumption (especially important in battery-powered laptops).
audible noise in dBA (although many still report it in bels, not decibels).
G-shock rating (surprisingly high in modern disks).
Inner Zone: from 44.2 MB/s to 74.5 MB/s.
Outer Zone: from 74.0 MB/s to 111.4 MB/s.
Random access time: from 5 ms to 15 ms.
Close-up of a hard disk head suspended above the disk platter together with its mirror image in the smooth surface of the magnetic platter.The hard disk's spindle system relies on air pressure inside the enclosure to support the heads at their proper flying height while the disk is in motion. A hard disk 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 1/2 mm in diameter), usually with a carbon filter on the inside (the breather filter, see below). If the air pressure is too low, there will not be enough lift for the flying head, the head will not be at the proper height, 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 10,000 feet (3,000 m). This does not apply to pressurized enclosures, like an airplane pressurized cabin. Modern disks include temperature sensors and adjust their operation to the operating environment.
Very high humidity for extended periods can cause accelerated wear of the heads and platters by corrosion. If the disk uses "Contact Start/Stop" (CSS) technology to park its heads on the platters when not operating, increased humidity can also lead to increased stiction (the tendency for the heads to stick to the platter surface). This can cause physical damage to the platter and spindle motor and can also lead to head crash. Breather holes can be seen on all disks — they usually have a warning sticker next to them, informing 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.
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. For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) will still result 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). Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured platters and heads. In most desktop and server disks, when powering down, the heads are moved to a landing zone, an area of the platter usually near its inner diameter (ID), where no data is stored. This area is called the CSS (Contact Start/Stop) zone. However, especially in old models, sudden power interruptions or a power supply failure can sometimes result in the device shutting down with the heads in the data zone, which increases the risk of data loss. In fact, it used to be procedure to "park" the hard disk before shutting down your computer. Newer disks are designed such that either a spring (at first) or (more recently) rotational inertia in the platters is used to safely park the heads in the case of unexpected power loss.
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. Also, most major hard disk and motherboard vendors now support self-monitoring, analysis, and reporting technology (S.M.A.R.T.), by which impending failures can be predicted, allowing the user to be alerted to prevent data loss.
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.Around 1995 IBM pioneered a technology where the landing zone is made by a precision laser process (Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in the ID landing zone, thus vastly improving stiction and wear performance. This technology is still widely in use today (2006). A few years after LZT, initially for mobile applications (i.e. laptop etc.), and later also for the other HDD types, IBM introduced "head unloading" technology, where the heads are lifted off the platters onto plastic "ramps" near the outer disk edge, thus eliminating the risk of stiction altogether and greatly improving non-operating shock performance. All HDD manufacturers use these two technologies to this day. Both have a list of advantages and drawbacks in terms of loss of storage space, relative difficulty of mechanical tolerance control, cost of implementation, etc.
IBM 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 motion sensor in the Thinkpad, internal hard disk heads automatically unload themselves into the parking zone to reduce the risk of any potential data loss or scratches made. Apple later also utilized this technology in their Powerbook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor.
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 reliably 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 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 Maxtor DiamondMax series of desktop hard disks are rated to 50,000 start-stop cycles. This means that no failures attributed to the head-platter interface were seen before at least 50,000 start-stop cycles during testing.
Access and interfaces
Hard disks are generally accessed over one of a number of bus types, including ATA (IDE, EIDE), Serial ATA (SATA), SCSI, SAS, IEEE 1394, USB, and Fibre Channel.
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 (which is still used on the common "1.44 MB" (1440 KiB) 3.5-inch floppy), 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 half, to 7.5 megabits per second; it also increased disk capacity by half.
Many ST-506 interface disks were only certified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk) were certified to run at the higher RLL data rate. In some cases, the disk was overengineered just enough to allow the MFM-certified model to run at the faster data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk 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 and controller; most of the time, however, 15 or 20 megabit ESDI disks weren't downward compatible (i.e. a 15 or 20 megabit disk wouldn't run on a 10 megabit controller). ESDI disks 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 5 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's internal data bus; however, many early disks 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 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) hard disks 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.
Disk families used in personal computers
Notable disk families include:
MFM (Modified Frequency Modulation) disks required that the controller electronics be compatible with the disk electronics.
RLL (Run Length Limited) disks were named after the modulation technique that made them an improvement on MFM. They required large cables between the controller in the PC and the hard disk, the disk did not have a controller, only a modulator/demodulator.
ESDI (Enhanced Small Disk Interface) was an interface developed by Maxtor to allow faster communication between the PC and the disk than MFM or RLL.
Integrated Drive Electronics (IDE) was later renamed to ATA, and then PATA.
The name comes from the way early families had the hard disk controller external to the disk. Moving the hard disk controller from the interface card to the disk helped to standardize interfaces, reducing cost and complexity.
The data cable was originally 40 conductor, but UDMA modes from the later disks requires using an 80 conductor cable (note that the 80 conductor cable still uses a 40 position connector.)
The interface changed from 40 pins to 39 pin. The missing pin acts as a key to prevent incorrect insertion of the connector, a common cause of disk and controller damage.
SCSI (Small Computer System Interface) was an early competitor with ESDI, originally named SASI for Shugart Associates. 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, and then Fibre Channel (FC) interface, and then more specifically the Fibre Channel Arbitrated Loop (FC-AL), connected SCSI hard disks using fibre optics. FC-AL is the cornerstone of storage area networks, although other protocols like iSCSI and ATA over Ethernet have been developed as well.
SATA (Serial ATA). The SATA data cable has only one data pair for the differential transmission of data to the device, and one pair for receiving from the device. That requires that data be transmitted serially. The same differential transmission system is used in RS485, LocalTalk, USB, Firewire,and differential SCSI. In 2005/2006 parlance, the 40 pin IDE/ATA is called "PATA" or parallel ATA, which means that there are 16 bits of data transferred in parallel at a time on the data cable.
SAS (Serial Attached SCSI). 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 for interacting with SAS
EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of DMA to transfer data between the disk and the computer, an improvement later adopted by the official ATA standards. DMA is used to transfer data without the CPU or program being responsible to transfer every word. That leaves the CPU/program/operating system to do other tasks while the data transfer occurs.
Acronym Meaning Description
SASI Shugart Associates System Interface Predecessor to SCSI
SCSI Small Computer System Interface Bus oriented that handles concurrent operations.
ST-412 Seagate interface
ST-506 Seagate interface (improvement over ST-412)
ESDI Enhanced Small Disk Interface Faster and more integrated than ST-412/506, but still backwards compatible
ATA Advanced Technology Attachment Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.
As of 2005, over 98% of the world's hard disks are manufactured by just a handful of large firms: Seagate, Maxtor (acquired by Seagate in May 2006), 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.
Dozens of former hard disk manufacturers have gone out of business, merged, or closed their hard disk divisions; as capacities and demand for products increased, profits became hard to find, and there were shakeouts 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. CMI's reputation never recovered, and they exited the hard disk business in 1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it was found that they had "cooked the books" 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 hard disks in India using a second hand factory. 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 hard disks.