hard disk

Magnetic 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.

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 5,000 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        Range of
  Interface   Method**  (Per sec)   Capacities

  SATA (IDE)  RLL       150-300MB    40GB-1.2TB

  PATA (IDE)  RLL       3-133MB     500MB-400GB

  SCSI        RLL       5-320MB      20MB-300GB


  Older Interfaces

  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 Measurements
Capacity is measured in bytes, and speed is measured by transfer rate in bytes per second (see above) and access time in milliseconds (ms). Hard disk access times range from 3 ms to about 15 ms, whereas CDs and DVDs range from 80 ms to 120 ms.

Non-Removable Internal Hard Disk

Hard disks use one or more metal or glass platters covered with a magnetic coating. Although there has been a variety of removable hard disks over the years, a computer's primary hard disks are fixed inside the drive. The entire unit is removed only to be replaced or repaired. In this drawing, the cover is removed.

First Hard Disk

Part computer, part tabulator, in 1956, IBM's RAMAC was the first machine with a hard disk, which was extraordinary technology of the times. Each of its 24" diameter platters held a whopping 100,000 characters (they were not bytes then) for a total of five million characters. (Images courtesy of International Business Machines Corporation. Unauthorized use not permitted.)

First Microcomputer Hard Disk

Seagate introduced the first hard disk for personal computers in 1979. At 5MB, the ST506 held 10 times as much as the RAMAC at a fraction of its size. (Image courtesy of Seagate Technology, Inc.)

Four Decades Later

Entry level these days, but in 1998, this Seagate drive's 47GB was impressive. Four decades of research and development let us store 100,000 times as much on the same platter surface. Even more impressive is that this much data is stored on one side of only one platter today. (Image courtesy of Seagate Technology, Inc.)

A Not-Even-One-Inch Drive

Toshiba introduced the first 0.85" hard drive and shipped 2GB and 4GB units in 2005. About the size of a postage stamp, it was named in that year's Guinness Book of Records as the world's smallest hard disk. Using perpendicular recording, Toshiba is expected to increase capacity to 8GB, targeting use in digital cameras, mobile phones, camcorders and tiny MP3 players. (Image courtesy of Toshiba Corporation.)

(storage)

hard disk - (In contrast to floppy disk) A magnetic disk data storage device where the disks are rigid and fixed to a central axle. They are usually packaged with associated read/write heads and electronics. Most hard disks are permanently connected to the drive (fixed disks) though there are also removable hard disks.

See magnetic disk.