Hard disk drive
A hard disk drive (HDD; also hard drive, hard disk, or disk drive)[2] is a device for storing and retrieving digital information, primarily computer data. It consists of one or more rigid (hence "hard") rapidly rotating discs (platters) coated with magnetic material, and with magnetic heads arranged to write data to the surfaces and read it from them.
Hard drives are classified as non-volatile, random access, digital, magnetic, data storage devices. Introduced by IBM in 1956, hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s.[3] They have maintained this position because of advances which have resulted in increased recording capacity, reliability, and speed, as well as decreased cost, allowing them to keep pace with ever more demanding requirements for secondary storage.[3]
[edit]Units of storage capacity
See also: Binary prefix
| Advertised capacity by manufacturer (using decimal multiples) | Expected capacity by consumers in class action (using binary multiples) | Reported capacity | |||
|---|---|---|---|---|---|
| Windows (using binary multiples) | Mac OS X 10.6+ (using decimal multiples) | ||||
| With prefix | Bytes | Bytes | Diff. | ||
| 100 MB | 100,000,000 | 104,857,600 | 4.86% | 95.4 MB | 100.0 MB |
| 100 GB | 100,000,000,000 | 107,374,182,400 | 7.37% | 93.1 GB, 95,367 MB | 100.00 GB |
| 1 TB | 1,000,000,000,000 | 1,099,511,627,776 | 9.95% | 931 GB, 953,674 MB | 1,000.00 GB, 1,000,000 MB |
The capacity of hard disk drives is given by manufacturers in megabytes(1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) orterabytes (1 TB = 1,000,000,000,000 bytes).[30][31] This numbering convention, where prefixes like mega- and giga- denote powers of 1,000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM,ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1,024.
When the unit prefixes like kilo- denote powers of 1,024 in the measure of memory capacities, the 1,024n progression (for n = 1, 2, ...) is as follows:[30]
- kilo = 210 = 1,0241 = 1,024,
- mega = 220 = 1,0242 = 1,048,576,
- giga = 230 = 1,0243 = 1,073,741,824,
The practice of using prefixes assigned to powers of 1,000 within the hard drive and computer industries dates back to the early days of computing.[32] By the 1970s million, mega and M were consistently being used in the powers of 1,000 sense to describe HDD capacity.[33][34][35] As HDD sizes grew the industry adopted the prefixes “G” for giga and “T” for tera denoting 1,000,000,000 and 1,000,000,000,000 bytes of HDD capacity respectively.
Likewise, the practice of using prefixes assigned to powers of 1,024 within the computer industry also traces its roots to the early days of computing[36] By the early 1970s using the prefix “K” in apowers of 1,024 sense to describe memory was common within the industry.[37][38] As memory sizes grew the industry adopted the prefixes “M” for mega and “G” for giga denoting 1,048,576 and 1,073,741,824 bytes of memory respectively.
Computers do not internally represent HDD or memory capacity in powers of 1,024; reporting it in this manner is just a convention.[39] Creating confusion, operating systems report HDD capacity in different ways. Most operating systems, including the Microsoft Windows operating systems use the powers of 1,024 convention when reporting HDD capacity, thus an HDD offered by its manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD. Apple's current OSes, beginning with Mac OS X 10.6 (“Snow Leopard”), use powers of 1,000 when reporting HDD capacity, thereby avoiding any discrepancy between what it reports and what the manufacturer advertises.
In the case of “mega-,” there is a nearly 5% difference between the powers of 1,000 definition and the powers of 1,024 definition. Furthermore, the difference is compounded by 2.4% with each incrementally larger prefix (gigabyte, terabyte, etc.) The discrepancy between the two conventions for measuring capacity was the subject of several class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled consumers[40][41] while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.[42]
In December 1998, an international standards organization attempted to address these dual definitions of the conventional prefixes by proposing unique binary prefixes and prefix symbols to denote multiples of 1,024, such as “mebibyte (MiB)”, which exclusively denotes 220 or 1,048,576 bytes.[43] In the over‑13 years that have since elapsed, the proposal has seen little adoption by the computer industry and the conventionally prefixed forms of “byte” continue to denote slightly different values depending on context.[44][45]
[edit]Current hard disk form factors
| Form factor | Width (mm) | Height (mm) | Largest capacity | Platters (max) | Per platter (GB) |
|---|---|---|---|---|---|
| 3.5″ | 102 | 19 or 25.4 | 4 TB[66][67][68][69] (2011) | 5 | 1000 GB |
| 2.5″ | 69.9 | 7,[70] 9.5,[71] 12.5,[72] or 15 | 2 TB[66][73][74] (2012) | 4 | 500 GB |
| 1.8″ | 54 | 5 or 8 | 320 GB[75] (2009) | 2 | 160 GB |
[edit]Obsolete hard disk form factors
| Form factor | Width (mm) | Largest capacity | Platters (max) | Per platter (GB) |
|---|---|---|---|---|
| 5.25″ FH | 146 | 47 GB[76] (1998) | 14 | 3.36 GB |
| 5.25″ HH | 146 | 19.3 GB[77] (1998) | 4[78] | 4.83 GB |
| 1.3″ | 43 | 40 GB[79] (2007) | 1 | 40 GB |
| 1″ (CFII/ZIF/IDE-Flex) | 42 | 20 GB (2006) | 1 | 20 GB |
| 0.85″ | 24 | 8 GB[80][81] (2004) | 1 | 8 GB |
[edit]Disk interface families used in personal computers
Historical bit serial interfaces connect a hard disk drive (HDD) to a hard disk controller (HDC) with two cables, one for control and one for data. (Each drive also has an additional cable for power, usually connecting it directly to the power supply unit). The HDC provided significant functions such as serial/parallel conversion, data separation, and track formatting, and required matching to the drive (after formatting) in order to assure reliability. Each control cable could serve two or more drives, while a dedicated (and smaller) data cable served each drive.
- ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
- ST412 was available in either MFM or RLL (Run Length Limited) encoding variants.
- Enhanced Small Disk Interface (ESDI) was an industry standard interface similar to ST412 supporting higher data rates between the processor and the disk drive.
Modern bit serial interfaces connect a hard disk drive to a host bus interface adapter (today typically integrated into the "south bridge") with one data/control cable. (As for historical bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.)
- 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, and 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 coppertwisted-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. A similar 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 a mechanically identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
Word serial interfaces connect a hard disk drive to a host bus adapter (today typically integrated into the "south bridge") with one cable for combined data/control. (As for all bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.) The earliest versions of these interfaces typically had a 8 bit parallel data transfer to/from the drive, but 16-bit versions became much more common, and there are 32 bit versions. Modern variants 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 standardized under the name AT Attachment, with the alias P-ATA or PATA (Parallel ATA) retroactively added upon introduction of the new variant Serial ATA. The original name reflected the integration of the controller with the hard drive itself. (That integration was not new with IDE, having been done a few years earlier with SCSI drives.) Moving the HDD controller from the interface card to the disk drive helped to standardize the host/contoller interface, reduce the programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin IDE/ATA connection 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. Progressively swifter versions of this standard ultimately added the requirement for an 80-conductor variant of the same cable, where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk. The interface for 80-conductor 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 eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks 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, Commodore Amiga, and Apple Macintosh computers through the mid-1990s, 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, SAS and Fibre Channel only. The range limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended (common mode) data transmission, but server class SCSI could use differential transmission, eitherlow voltage differential (LVD) or high voltage differential (HVD). ("Low" and "High" voltages for differential SCSI are relative to SCSI standards and do not meet the meaning of low voltage and high voltage as used in general electrical engineering contexts, as apply e.g. to statutory electrical codes; both LVD and HVD use low voltage signals (3.3 V and 5 V respectively) in general terminology.)
| Acronym or abbreviation | Meaning | Description |
|---|---|---|
| SASI | Shugart Associates System Interface | Historical predecessor to SCSI. |
| SCSI | Small Computer System Interface | Bus oriented that handles concurrent operations. |
| SAS | Serial Attached SCSI | Improvement of SCSI, uses serial communication instead of parallel. |
| ST-506 | Seagate Technology | Historical Seagate interface. |
| ST-412 | Seagate Technology | Historical Seagate interface (minor improvement over ST-506). |
| ESDI | Enhanced Small Disk Interface | Historical; backwards compatible with ST-412/506, but faster and more integrated. |
| ATA [(PATA)Parallel Advanced Technology Attachment] | Advanced Technology Attachment, | Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations. |
| SATA | Serial ATA | Modification of ATA, uses serial communication instead of parallel. |
[edit]Recovery of data from failed drive
Data from a failed drive can sometimes be partially or totally recovered if the platters' magnetic coating is not totally destroyed. Specialised companies carry out data recovery, at significant cost, by opening the drives in a clean room and using appropriate equipment to read data from the platters directly. If the electronics have failed, it is sometimes possible to replace the electronics board, though often drives of nominally exactly the same model manufactured at different times have different, incompatible, circuit boards.
Sometimes operation can be restored for long enough to recover data. Risky techniques are justifiable if the drive is otherwise dead. If a drive is started up once it may continue to run for a shorter or longer time but never start again, so as much data as possible is recovered as soon as the drive starts. A 1990s drive that does not start due to stiction can sometimes be started by tapping it or rotating the body of the drive rapidly by hand. Another technique which is sometimes known to work is to cool the drive, in a waterproof wrapping, in a domestic freezer. There is much useful information about this in blogs and forums,[100] but professionals also resort to this method with some success.[101]
External removable drives
External removable hard disk drives[114] typically connect via USB. Plug and play drive functionality offers system compatibility, and features large storage options and portable design. External hard disk drives are available in 2.5" and 3.5" sizes, and as of March 2012 their capacities generally range from 160GB to 2TB. Common sizes are 160GB, 250GB, 320GB, 500GB, 640GB, 1TB, and 2TB.[115][116]
External hard disk drives are available as preassembled integrated products, or may be assembled by combining an external enclosure (with USB or other interface) with a separately-purchased drive.
Features such as biometric security or multiple interfaces are available at a higher cost.[117]
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