Drives, Controllers & Storage

A hard disk drive (HDD, also commonly shortened to hard drive and formerly known as a fixed disk) is a digitally encoded non-volatile storage device which stores data on rapidly rotating platters with magnetic surfaces. more...

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Strictly speaking, "drive" refers to an entire unit containing multiple platters, a read/write head assembly, driver electronics, and motor while "hard disk" (sometimes "platter") refers to the storage medium itself.

Hard disks were originally developed for use with computers. In the 21st century, applications for hard disks have expanded beyond computers to include video recorders, audio players, digital organizers, and digital cameras. In 2005 the first cellular telephones to include hard disks 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, hardware such as network attached storage (NAS) devices, and systems such as storage area networks (SANs) for efficient access to large volumes of data.

Technology

Hard disks 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 hard disk 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 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 magnetic surface of each platter is divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's hard disks each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localised magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early hard disks used the same inductor that was used to read the data as an electromagnet to create this field. Later, metal in Gap (MIG) heads were used, and today thin film heads are common. With these later technologies, the read and write head are separate mechanisms, but are on the same actuator arm.

Read more at Wikipedia.org