A hard disk drive, hard disk or hard drive is a non-volatile data storage device that stores data on rapidly rotating platters with magnetic surfaces.

Normally the word "drive" refers to the equipment used to process data stored in its recording medium, such as an optical disk drive and its optical disk or a floppy drive and its floppy disk. Early hard drives had removable media, but today a hard drive is typically a sealed unit with fixed media.

Technology

A hard drive records data by magnetising ferromagnetic material directionally to represent either a 0 or a 1 binary digit. It reads the data back by detecting the magnetisation of the material. A typical hard drive design consists of a spindle that holds one or more flat circular disks called platters, onto which the data is recorded.

The platters are spun at very high speeds. Information is written to a platter as it rotates past devices called read/write heads that operate very close over the magnetic surface. The read/write head is used to detect and modify the magnetisation 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 in an arc across the platters as they spin, allowing each head to access almost the entire surface of each platter.

The magnetic surface of each platter is conceptually divided into many small magnetic regions, each of which is used to encode a single binary unit of data.

Disk Structure

The physical structure of a hard drive is as follows:

The physical properties of a hard drive are as follows:

platter	A hard drive contains one or more flat disks that are used to store data. These disks are called platters. They are composed of two main substances: a substrate material that forms the bulk of the platter and gives it structure and rigidity, and a magnetic media coating which stores the magnetic impulses that represent the data.
read/write head	Data is stored to and retrieved from the surface of a platter by a device called a read/write head. A platter has two sides and thus two surfaces on which data can be stored; usually there are 2 heads per platter - one on each side. A head is positioned on an actuator assembly which moves to enable the head to access different areas of the recording medium (the platter).
track	A track is one of the thin concentric circular strips on a platter surface which actually contain the magnetic regions of data. A track is the intersection of a head and a cylinder.
cylinder	A cylinder comprises all the tracks (on every usable platter surface) that can be accessed by the read/write heads while the actuator assembly remains stationary.
geometric sector	Geometrically, a sector is defined as a “pie-slice” part of a circle, or the area between two radii and the connecting arc of the circle. Data stored in a hard drive is referenced by the intersection of a track and a geometric sector, known as a data sector.
data sector	In the context of data storage, a data sector, often called a block, is a subdivision of a track. It refers to the intersection of a track and a geometric sector. Each sector stores a fixed amount of data. Typical formatting of disk media provides space for 512 bytes of data per sector on a magnetic disk.

Addressing Schemes

CHS

Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data in a hard drive.

Using the CHS method, each block is addressed by means of a tuple which defines the cylinder, head, and geometric sector at which it appears in the drive.

LBA

Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on data storage devices . The term LBA can mean either the address or the block to which it refers. Logical blocks in modern computer systems are typically 512 or 1024 bytes each.

LBA is a particularly simple addressing scheme; blocks are located by an index, with the first block being LBA = 0, the second LBA = 1 and so on. The LBA scheme replaces earlier schemes, such as CHS, which exposed the physical details of the storage device to the software of the operating system.

Access and Interfaces

Hard drives are accessed over one of a number of bus types.

FireWire/IEEE 1394 and USB(1.0/2.0) hard drives are external units containing generally ATA or SCSI drives. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself. However, USB is a point to point network and does not allow for daisy-chaining. USB hubs are used to increase the number of available ports and are used for devices that do not require charging since the current supplied by hubs is typically lower than what is available from the built-in USB ports.

Interface Families

Notable families of interfaces include:

word serial interfaces	IDE, ATA, PATA	Integrated Drive Electronics, Advanced Technology Attachment, Parallel ATA The IDE interface integrates the disk controller into the device. It is incapable of concurrent operations. IDE is also known as ATA, with the alias PATA added retrospectively with the introduction of the new variant SATA (Serial ATA). The original name IDE reflected the integration of the hard drive controller into drive itself, which was not found in earlier drives. Moving the controller from the interface card to the drive helped to standardise interfaces and to reduce cost and complexity.
	EIDE	Enhanced IDE EIDE was an unofficial update to the original IDE standard. The key improvement was the use of direct memory access (DMA) to transfer data between disk and computer without involving 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 each byte, freeing it to process other tasks during data transfer.
	SCSI	Small Computer System Interface SCSI is a bus oriented interface that is capable of concurrent operations. It was originally named SASI for Shugart Associates System Interface, and was an early competitor of IDE.
bit serial interfaces	FC	Fibre Channel FC is a serial protocol. It is used for purposes other than functioning as a disk interface, and is a key component of storage area networks. Other protocols have also been developed for similar purposes, such as iSCSI and ATA over Ethernet. Drives usually use copper twisted-pair cables for Fibre Channel. Fibre optics are normally used for larger devices such as servers or disk array controllers.
	SATA	Serial ATA SATA is a serial interface. Its data cable has one data pair for differential transmission of data to the device and one pair for differential receiving from the device. This requires serial data transmission.
	SAS	Serial Attached SCSI SAS is a serial communication protocol compatible with SATA allowing for high speed data transfers. SAS uses the same data and power connectors as standard 3.5" SATA1/SATA2 hard drives, 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.

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