Hard Disk

A hard disk drive (HDD is a data storage device used for storing and retrieving digital information using rapidly rotating disks (platter) coated with magnetic material. An HDD retain its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating disks (platters) with magnetic heads  arranged on a moving actuator arm to read and write data to the surfaces.
Introduced by IBM in 1956 HDDs became the dominant secondary storage device for general purpose computers by the early 1960s. Continuously improved, HDDs have maintained this position into the modern era of severs and personal computers. More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital. Worldwide revenues for HDD shipments are expected to reach US $33 billion in 2013, a decrease of approximately 12% from US $37.8 billion in 2012.
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 gigabyte (GB; where 1 gigabyte = 1 billion byte). 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.