Most consumer HDDs operate at a speed of 7,200 rpm. The platters in contemporary HDDs are spun at speeds varying from 4,200 rpm in energy-efficient portable devices, to 15,000 rpm for high-performance servers. For reference, a standard piece of copy paper is 0.07–0.18 millimetre (70,000–180,000 nm). The platters are made from a non-magnetic material, usually aluminium alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection.
User data is encoded using an encoding scheme, such as run-length limited encoding, which determines how the data is represented by the magnetic transitions.Ī typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. The data is read from the disk by detecting the transitions in magnetization. Sequential changes in the direction of magnetization represent binary data bits. Magnetic cross section & frequency modulation encoded binary dataĪn HDD records data by magnetizing a thin film of ferromagnetic material on a disk.
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