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A hard disk drive (HDD), hard disk, hard drive or fixed disk is a data storage device used for storing and retrieving digital information using one or more rigid ("hard") rapidly rotating disks (platters) coated with magnetic material. The platters are paired with magnetic heads arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order rather than sequentially. HDDs retain stored data even when powered off. Introduced by IBM in 1956,〔 HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s. Continuously improved, HDDs have maintained this position into the modern era of servers and personal computers. More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital. Worldwide disk storage revenues were US $32 billion in 2013, down 3% from 2012. The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of : a 1-terabyte (TB) drive has a capacity of gigabytes (GB; where 1 gigabyte = bytes). Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Performance is specified by the time required to move the heads to a track or cylinder (average access time) plus the time it takes for the desired sector to move under the head (average latency, which is a function of the physical rotational speed in revolutions per minute), and finally the speed at which the data is transmitted (data rate). The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables. , the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs), which have higher data transfer rates, better reliability, and significantly lower latency and access times, but HDDs remain the dominant medium for secondary storage due to advantages in price per bit and per-device recording capacity. However, SSDs are replacing HDDs where speed, power consumption and durability are more important considerations.〔Hutchinson, Lee. (2012-06-25) (How SSDs conquered mobile devices and modern OSes ). Ars Technica. Retrieved on 2013-01-07.〕 == History == (詳細はtransaction processing computer and were developed for use with general-purpose mainframe and minicomputers. The first IBM drive, the 350 RAMAC, was approximately the size of two refrigerators and stored five million six-bit characters (3.75 megabytes) on a stack of 50 disks.〔(【引用サイトリンク】url=https://www-03.ibm.com/ibm/history/exhibits/storage/storage_350.html )〕 The IBM 350 RAMAC disk storage unit was superseded by the IBM 1301 disk storage unit,〔(【引用サイトリンク】title=IBM Archives: IBM 1301 disk storage unit )〕 which consisted of 50 platters, each about 1/8-inch thick and 24 inches in diameter.〔(【引用サイトリンク】title=DiskPlatter-1301 )〕 Whereas the IBM 350 used two read/write heads, pneumatically actuated〔 and moving through two dimensions, the 1301 was one of the first disk storage units to use an array of heads, one per platter, moving as a single unit. Cylinder-mode read/write operations were supported, while the heads flew about 250 micro-inches above the platter surface. Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes. Access time was about 200 milliseconds. In 1962, IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called "fixed disk" drives. Some high-performance HDDs were manufactured with one head per track (e.g. IBM 2305) so that no time was lost physically moving the heads to a track.〔Microsoft Windows NT Workstation 4.0 Resource Guide 1995, Chapter 17 – Disk and File System Basics〕 Known as fixed-head or head-per-track disk drives they were very expensive and are no longer in production.〔Computer Organization and Design, 2nd Ed., P. Pal Chaudhuri, 2006, p. 635〕 In 1973, IBM introduced a new type of HDD codenamed "Winchester". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable media concept and returned to non-removable platters. Like the first removable pack drive, the first "Winchester" drives used platters in diameter. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages. Drives with non-removable eight-inch platters appeared, and then drives that used a form factor (a mounting width equivalent to that used by contemporary floppy disk drives). The latter were primarily intended for the then-fledgling personal computer (PC) market. As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers. Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10 MB HDD, and soon thereafter internal HDDs proliferated on personal computers. External HDDs remained popular for much longer on the Apple Macintosh. Every Macintosh made between 1986 and 1998 had a SCSI port on the back, making external expansion simple; also, older compact Macintosh computers did not have easily accessible hard drive bays (or in the case of the Macintosh 128K, Macintosh 512K, and Macintosh Plus, any hard drive bay at all), so on those models external SCSI disks were the only reasonable option. The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013. Driven by ever increasing areal density since their invention, HDDs have continuously improved their characteristics; a few highlights are listed in the table above. At the same time, market application expanded from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hard disk drive」の詳細全文を読む スポンサード リンク
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