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

(storage)
(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, which 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 spins inside the drive.

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

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. For a summary of memory and storage types, see storage vs. memory.

Capacity and Speed
Capacity is measured in bytes, and the largest drives hold up to 10 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 SSDs (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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References in periodicals archive ?
Related-Technology Firms were diversified concerns that entered by adapting magnetic data-recording technologies they had developed in other product-market contexts to rigid disk drive products.
Clearly, the rigid disk drive industry was not an industry pioneered by classic venture capital-backed Silicon Valley start-up firms.
Without exception, the start-ups that grew to dominate the world industry were focused exclusively on manufacturing rigid disk drives--they made no other products.
Table 4 Trend Toward Less Vertical Integration in the Disk Drive Industry Extent of Vertically IBM Control Seagate Conner Quantum Integrated Activities Data Peripherals (X denotes the firm had integrated into that activity) Date Entered Industry 1956 1962 1980 1986 1987 Approximate 1991 $4,500 (a) $2,600(a) $1,600 $1,100 Rigid Disk Drive Sales Computer Manufacturing X X Disk Drive Design X X X X X Disk Drive Assembly X X X X Head Manufacturing X X X Disk Manufacturing X X X Research in New Head X X (b) and Disk Technologies a Includes sales of the former Control Data Corporation disk drive operation, which logged revenues of approximately $1 billion in each of the several years prior to its acquisition by Seagate.
From 1981 to 1986, when over sixty firms entered the rigid disk drive industry, only five of them (all commercial failures) attempted to do so using thin-film heads as a source of performance advantage in their initial products.
Rigid disks generally pack greater areal densities than tapes.
2] areal densities achievable with rigid disks, but it shines in the volume density (kb/mm[sub.3]) category.
of included shaft subsegment Length, Inner diameter, Outer diameter, segment of the rotor mm mm mm 1 1 20 40 100 2 1 20 40 100 2 20 42 120 3 45 40 90 3 1 45 30 90 2 54 30 100 3 5 30 80 4 100 30 80 4 1 100 30 80 2 30 30 60 5 1 30 30 60 2 10 30 65 6 1 10 30 65 Table 2 Parameters of the rigid disks of the dynamic model Young's Width, Inner Outer Density, modulus, No.
Bhushan, "Studies on degradation mechanisms of lubricants for magnetic thin-film rigid disks," Proceedings of the Institution ofMechanical Engineers J: Journal ofEngineering Tribology, vol.