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hard disk |
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hard diskMagnetic storage medium for a microcomputer. Hard disks are flat, circular plates made of aluminum or glass and coated with a magnetic material. Hard disks for personal computers can store up to several gigabytes (billions of bytes) of information. Data are stored on their surfaces in concentric tracks. A small electromagnet, called a magnetic head, writes a binary digit (1 or 0) by magnetizing tiny spots on the spinning disk in different directions and reads digits by detecting the magnetization direction of the spots. A computer's hard drive is a device consisting of several hard disks, read/write heads, a drive motor to spin the disks, and a small amount of circuitry, all sealed in a metal case to protect the disks from dust. In addition to referring to the disks themselves, the term hard disk is also used to refer to the whole hard drive. hard disk The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Most hard disks are "fixed disks," which have platters that reside permanently in the drive. Almost all computers have an internal hard disk, and external units can be plugged in for additional storage or backup.The other type of hard disk is a "removable disk" encased in a cartridge, allowing data to be ejected from the drive for external storage or transfer to another party. Before high-speed Internet connections were common, removable SyQuest, Jaz and Zip cartridges were routinely shipped via the post office (see removable disk). Three Major Categories: PATA, SATA and SCSI Most hard disks are Parallel ATA (PATA), Serial ATA (SATA) or SCSI. SCSI drives have traditionally been found on servers and high-performance workstations and were the first drives used in fault-tolerant RAID systems. Today, ATA drives are widely used for RAID arrays. See IDE, PATA, SATA, SCSI and RAID. Hard drives are low-level formatted at the factory, which records the original sector identification on the platters (see format program). See hard disk defect management. Fast Rotation Hard disks provide fast retrieval, because they rotate constantly at high speed from 4,000 to 15,000 RPM. Either to preserve battery life in laptops or to promote longevity, hard disks can be configured to turn off after a defined period of inactivity. It Started in the Mid-1950s In 1956, IBM introduced the RAMAC hard disk with platters two feet in diameter that held the equivalent of 100,000 bytes. In the 1980s, desktop computer hard disks were introduced with 5MB using 5.25" platters (see ST506). Today's entry-level drives have at least 8,000 times more capacity. Platter size was reduced to 3.5" for desktops, 2.5" for laptops and 1" for handhelds. In 2004, Toshiba introduced the 0.85" drive (see below). See magnetic disk, floppy disk, Microdrive, drop protection and CAV.
TYPES OF HARD DISKS
Transfer
Type of Encoding Rate Maximum
Interface Method** (Per sec) Capacity
SATA (IDE) RLL 150-300MB 2TB
PATA (IDE) RLL 3-133MB 1TB
SCSI RLL 5-320MB 300GB
SAS RLL 375-750MB 2TB
Capacity
Older Interfaces Range
IPI RLL 10-25MB 200MB-3GB
ESDI RLL 1-3MB 80MB-2GB
SMD RLL 1-4MB 200MB-2GB
IDE RLL 1-8MB 40MB-1GB
ST506 RLL RLL 937KB 30MB-200MB
ST506 MFM 625KB 5MB-100MB
** Most disks use RLL, but encoding methods are
not prescribed by all interfaces.
hard disk a disk of rigid magnetizable material that is used to store data for computers: it is permanently mounted in its disk drive and usually has a storage capacity of a few gigabytes hard disk [′härd ¦disk] (computer science) A magnetic disk made of rigid material, providing high-capacity random-access storage.
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