page_title: Hard disk drive
section_title: Access and interfaces
hierarchical_section_title: Hard disk drive / Access and interfaces
caption_attribution_description: English: Inner view of a Seagate 3.5 inches hard disk drive Medalist ST33232A model manufactured in Malaysia in 1998. Parallel ATA interface, ultra DMA mode 2, I/O data-transfer rate 33.3 MB/s max, 3,227 MB, 3 platters, 6 physical read/write heads, spindle speed 4,500 RPM, cache buffer 128 kB, MTBF 300,000 hours, dimensions 146.8 x 102.4 x 26.2 mm, weight 540 grams. Français&#160;: Vue interne d'un disque dur Seagate de 3,5 pouces, modèle Medalist ST33232A, fabriqué en Malaisie en 1998. Interface Parallel ATA, Ultra DMA mode 2, taux de transfert 33,3 MB/s max, capacité 3&#160;227 MB, 3 plateaux, 6 têtes physiques d'écriture/lecture, vitesse de rotation 4&#160;500 tr/min, cache 128 kB, MTBF 300&#160;000 heures, dimensions 146,8 x 102,4 x 26,2 mm, poids 540 g.
context_page_description: A hard disk drive, hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that uses magnetic storage to store and retrieve digital data using one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data even when powered off.
  Introduced by IBM in 1956, HDDs were the dominant secondary storage device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and personal computers, though personal computing devices produced in large volume, like cell phones and tablets, rely on flash products. More than 224 companies have produced HDDs historically, though after extensive industry consolidation most units are manufactured by Seagate, Toshiba, and Western Digital. HDDs dominate the volume of storage produced for servers.
context_section_description: Current hard drives connect to a computer over one of several bus types, including parallel ATA, Serial ATA , SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Some drives, especially external portable drives, use IEEE 1394, or USB. All of these interfaces are digital; electronics on the drive process the analog signals from the read/write heads. Current drives present a consistent interface to the rest of the computer, independent of the data encoding scheme used internally, and independent of the physical number of disks and heads within the drive.
  Typically a DSP in the electronics inside the drive takes the raw analog voltages from the read head and uses PRML and Reed–Solomon error correction to decode the data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.
  Modern interfaces connect the drive to the host interface with a single data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit. Older interfaces had separate cables for data signals and for drive control signals.
  Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was standard on servers, workstations, Commodore Amiga, Atari ST and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family disks. The length limit of the data cable allows for external SCSI devices.
  Integrated Drive Electronics (IDE), later standardized under the name AT Attachment (ATA, with the alias PATA (Parallel ATA) retroactively added upon introduction of SATA) moved the HDD controller from the interface card to the disk drive. This helped to standardize the host/controller 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 led to an   "ultra DMA" (UDMA) mode using an 80-conductor cable with additional wires to reduce cross talk at high speed.
  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.
  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 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 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 HDDs. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
  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
caption: Inner view of a 1998 Seagate HDD that used Parallel ATA interface
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