hard disk

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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
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

hard disk

[′härd ¦disk]
(computer science)
A magnetic disk made of rigid material, providing high-capacity random-access storage.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

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.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

hard disk

The primary computer storage medium until the 2000s, when solid state drives began replacing them in greater quantity. The first hard disk was introduced by IBM in 1956 (see RAMAC).

A hard disk is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Although the terms "hard disk" and "hard drive" are used synonymously; technically, the disk platters spin inside the drive.

All computers used to have an internal hard disk for storage; however, today, storage can be solid state, especially in laptops (see SSD). For more storage, external hard disks can be plugged into USB or an eSATA port.

Hard disks are "fixed disks," which means the platters reside permanently in the drive. In the past, removable cartridges for backup and transport contained the platters (see removable disk).

Storage... Not Memory
Hard disks are not the computer's main memory. Disks store programs and data until deliberately deleted by the user, but memory (RAM) is a temporary workspace. To learn how this workspace is used to process data, see memory and storage vs. memory.

Capacity and Speed
Capacity is measured in bytes, and the largest drives hold up to 18 terabytes. Speed is measured by transfer rate in megabytes per second as well as latency: how long it takes to begin transferring data, typically 3 to 15 milliseconds (ms). By comparison, CDs/DVDs take 80 to 120 ms.

The platters rotate constantly at thousands of RPM; however, to preserve battery, laptops default to powering them down after a set period of no activity. This mechanical action is why hard disks are slowly fading into history and being replaced with solid state drives (see SSD). See hard drive capacity, access time and transfer rate.

Disks Come Pre-Formatted
Hard disks are pre-formatted at the factory, which divides the platters into identifiable sectors. For more details on disk structure, see magnetic disk, format program, hard disk defect management and drop protection.

Hard Disk Types
Over the years, several kinds of hard disks have emerged. Today, SATA drives are the most common, although SAS drives are also used. For more details, see SATA, SAS, SCSI and hard disk interfaces.

Non-Removable Internal Hard Disk
Hard disks use one or more metal or glass platters covered with a magnetic coating. In this drawing, the cover is removed.

First Hard Disk Computer (1956)
Extraordinary technology, IBM's RAMAC was part computer, part tabulator and the first computer with a hard disk. (Image courtesy of IBM.)

Five Megabytes Total
Each of the RAMAC's 50 platters two feet in diameter held a whopping 100,000 characters. Today, the capacity of all 50 would fit on the head of a pin. (Image courtesy of IBM.)

First Personal Computer Hard Disk (1979)
In 1979, Seagate introduced the first hard disk for personal computers with 5MB on two 5.25" platters, the same capacity as all 50 platters in the RAMAC. Today, platters are 3.5" for desktops and 2.5" and 1.8" for laptops. See ST506. (Image courtesy of Seagate Technology, Inc.)

Four Decades After RAMAC (1998)
This 47GB Seagate drive held 100,000 times as much data as the RAMAC. Today, 20 times more storage is available on a flash drive (see USB drive). (Image courtesy of Seagate Technology, Inc.)

World's Smallest (2005)
The size of a postage stamp, the Microdrive was a less-than-one-inch hard disk for mobile devices with up to 8GB of storage (see Microdrive).
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