Working of Hard Disk Drive

An HDD records data by magnetizing a thin film of ferromagnetic material on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. 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. A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. 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. For reference, a standard piece of copy paper is 0.07–0.18 millimetres (70,000–180,000 nm). Information is written to and read from a platter as it rotates past devices called read-and-write heads that operate very close (often tens of nanometers) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it.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.^[24]The first HDDs spun at 1,200 rpm^[3] and, for many years, 3,600 rpm was the norm.^[25] As of December 2013, the platters in most consumer-grade HDDs spin at either 5,400 rpm or 7,200 rpm

.