Contents
Technology
Capacity and access speed
• Capacity measurements
History
Hard disk drive characteristics
Integrity
• Landing zones
Access and interfaces
• Disk families used in personal computers
Manufacturers
References
Hard disk drive
An IBM hard disk drive with the metal cover removed. The platters are highly reflective.
The inside of a hard disk drive displaying the actuator arm traveling over the top platter.
A hard disk drive (HDD), commonly referred to as a hard drive or hard disk,[1] is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media.[2]
HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras, and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia. The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.
Technology
A cross section of the magnetic surface in action. In this case the binary data encoded using frequency modulation
.HDDs record data by magnetizing a magnetic material in a pattern that represents the data. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually glass or aluminum, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.
The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that fly very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins.
The inside of a hard disk drive with the disk(s) and spindle motor hub removed. To the left of center is the actuator arm. A read-write head is at the end of the arm. In the middle the internal structure of the drive's spindle motor can be seen.
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