Term:Hard Disk Drive [HDD]

An HDD, or Hard Disk Drive, is data storage made up of magnetic spinning disks.

An HDD is typically a small box inside your computer that holds several metal disks coated with a magnetic  material. They are not unlike a video or audio tape or the strip on the back of a credit card. The disk spins at a fairly high rate of speed, and special “heads” can read, or record, a pattern of bits (1s and 0s) on the magnetic surface. Those bits are assembled into bytes, which in turn are the files that you save, receive, or create.

Disks do not lose what’s on them when you turn off the power.

See also: Solid State Disks (SSD)

Synonyms:
hard disk, hard disk drive, HD, HDD
Hard Disk Drive (Wikipedia)

Hard disk drive
IBM 350 RAMAC.jpg
Partially disassembled IBM 350 (RAMAC)
Date inventedDecember 24, 1954; 66 years ago (1954-12-24)
Invented byIBM team led by Rey Johnson
Internals of a 2.5-inch laptop hard disk drive
A disassembled and labeled 1997 HDD lying atop a mirror
An overview of how HDDs work

A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage and 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. Modern HDDs are typically in the form of a small rectangular box.

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 memory storage devices. 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 (exabytes per year) for servers. Though production is growing slowly (by exabytes shipped), sales revenues and unit shipments are declining because solid-state drives (SSDs) have higher data-transfer rates, higher areal storage density, better reliability, and much lower latency and access times.

The revenues for SSDs, most of which use NAND flash memory, slightly exceed those for HDDs. Flash storage products had more than twice the revenue of hard disk drives as of 2017. Though SSDs have four to nine times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small size, high capacity and durability are important. Cost per bit for SSDs is falling, and the price premium over HDDs has narrowed.

The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion (109) bytes). Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Also there is confusion regarding storage capacity, since capacities are stated in decimal gigabytes (powers of 1000) by HDD manufacturers, whereas the most commonly used operating systems report capacities in powers of 1024, which results in a smaller number than advertised. Performance is specified by the time required to move the heads to a track or cylinder (average access time) adding the time it takes for the desired sector to move under the head (average latency, which is a function of the physical rotational speed in revolutions per minute), and finally the speed at which the data is transmitted (data rate).

The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial Attached SCSI) cables.


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