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The primary characteristics of an HDD are its capacity and performance. As of 2015, HDD production (exabytes per year) and areal density are growing, although unit shipments are declining.
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More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital.
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Continuously improved, HDDs have maintained this position into the modern era of servers and personal computers. Introduced by IBM in 1956, HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s. HDDs are a type of non-volatile memory, retaining stored data even when powered off. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially. The platters are paired with magnetic heads arranged on a moving actuator arm, which read and write data to the platter surfaces.
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A hard disk drive (HDD), hard disk, hard drive or fixed disk is a data storage device used for storing and retrieving digital information using one or more rigid ("hard") rapidly rotating disks (platters) coated with magnetic material.
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