Hard disk drive

A data-storage device that reads and writes information on rapidly rotating magnetic disks using movable read–write heads. It retains data without power.

Magnetic recording: The storage of information by changing magnetic states in a material. A drive encodes bits as magnetic patterns on its disk surfaces.
Disk track: A concentric circular path on a disk surface where magnetic data is recorded. Tracks divide each platter surface into paths followed by the rotating medium.
Solid-state drive: A data-storage device that uses flash memory and has no moving mechanical parts. It contrasts with hard drives in access latency, durability, and storage technology.
IBM 305 RAMAC: An IBM computer system introduced in 1956 with the first commercial hard disk drive. Its disk storage unit brought random-access magnetic storage into commercial computing.
Disk platter: A rigid circular disk whose magnetic surface stores data in a hard disk drive. The platter spins beneath the read–write heads and carries the recorded tracks.
Cylinder (disk drive): A set of tracks at the same radius across a drive’s platter surfaces. Cylinder geometry describes aligned tracks reachable without moving the actuator arm.
Floppy disk: A removable magnetic storage medium made from a flexible disk inside a protective casing. It uses magnetic disk recording but is flexible, removable, and far lower in capacity.
IBM 350 disk storage unit: The disk storage component of the IBM 305 RAMAC, holding data on stacked magnetic disks. It was the pioneering commercial drive design that established the basic form of disk storage.
Read–write head: A component that senses magnetic patterns on a disk and changes them during writing. The head reads or alters data while flying just above the platter.
Rotational latency: The wait for a rotating disk to bring a desired sector beneath its read–write head. It is a major part of the delay after a drive positions its head.

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