Definition of a hard disk

A hard disk drive (HDD) is a digital data storage device that writes and reads data via magnetization changes of a magnetic storage disk along concentric tracks. Computer hard disk drives have become a standard data storage component of modern computer systems and are used in other modern consumer electronics as well. Their proliferation can be directly attributed to their low cost, high storage capacity and high reliability, in addition to wide availability, low power consumption, high data transfer speeds and decreasing physical size.

A hard disk drive contains rigid, disk-shaped platters usually constructed of aluminium or glass. Unlike floppy disks, the platters cannot bend or flex — hence the term hard disk. In most hard disk drives, the platters cannot be removed; for that reason, IBM calls them fixed disk drives. Hard disk drives used to be called Winchester drives. This term dates back to the 1960s, when IBM developed a high-speed hard disk drive that had 30M of fixed-platter storage and 30M of removable-platter storage. The drive had platters that spun at high speeds and heads that floated over the platters while they spun in a sealed environment. That drive, the 30-30 drive, soon received the nickname Winchester after the famous Winchester 30-30 rifle. After that time, drives that used a high-speed spinning platter with a floating head also became known as Winchester drives. The term

has no technical or scientific meaning; it is a slang term, and is considered synonymous with hard disk.

A hard drive uses spinning disks with heads that move over the disks and store data in tracks and sectors. The mechanism for reading and writing data on a disk is called an access arm. A track is a concentric ring of information, which is divided into individual sectors that normally store 512 bytes each. The nearer to the circumference of the magnetic disk, the higher the track recording density can be made.

Hard disks usually have multiple platters, each with two sides on which data can be stored. Most drives have at least two or three platters, resulting in four or six sides, and some drives have up to 11 or more platters. The identically positioned tracks on each side of every platter together make up a cylinder. A hard disk drive normally has one head per platter side, and all the heads are mounted on a common carrier device, or rack. The heads move in and out across the disk in unison; they cannot move independently because they are mounted on the same rack.

The heads in most hard disks do not and should not touch the platters during normal operation. When the heads are powered off, however, they land on the platters as they stop spinning. While the drive is on, a very thin cushion of air keeps each head suspended a short distance above or below the platter. If the air cushion is disturbed by a particle of dust or a shock, the head may come into contact with the platter spinning at full speed. When contact with the spinning platters is forceful enough to do damage, the event is called a head crash. The result of a head crash may be anything from a few lost bytes of data to a totally trashed drive. Most drives have special lubricants on the platters and hardened surfaces that can withstand the daily “takeoffs and landings” as well as more severe abuse.

Magnetic drives, such as hard disk drives, operate by using electromagnetism. This basic principle of physics states that as electric current flows through a conductor, a magnetic field is generated around the conductor. This magnetic field then can influence magnetic material in the field. When the direction of the flow of electric current is reversed, the magnetic field’s polarity also is reversed. An electric motor uses electromagnetism to exert pushing and pulling forces on magnets attached to a rotating shaft.

Another effect of electromagnetism is that if a conductor is passed through a changing magnetic field, an electrical current is generated. As the polarity of the magnetic field changes, so does the direction of the electric current flow. For example, a type of electrical generator used in automobiles, called an alternator, operates by rotating electromagnets past

coils of wire conductors in which large amounts of electrical current can be induced. The two-way operation of electromagnetism makes it possible to record data on a disk and read that data back later.

The overwhelming majority of the data necessary to operate the hardware and software of the computer is stored on the hard disk drive. The larger the capacity of the hard disk drive, the more data the computer can store. The storage capacity of hard drive is measured in megabytes, gigabytes, or terabytes.

The flow of data to and from the hard disk is managed by a collection of electronic circuits called the hard disk controller. The controller can be built into the disk drive or it can be a separate board in an expansion slot. For personal computers two types of controllers are common, IDE and SCSI. Integrated drive electronics (IDE) controllers can operate one or two hard disk drives. Small computer system interface, or SCSI (pronounced scuzzy), controllers can support multiple disk drives or mix of other SCSI- compatible devices.

Comments:

proliferation швидке збільшення

access доступ

circumference коло

head crash аварія голівок//непередбачуване зіткнення рухомих

магнітних голівок із поверхнею жорсткого диска, що призводить до псування даних або відмови на­копичувана

to exert здійснювати

alternator альтернатор, генератор змінного струму