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A hard disk drive (HDD), hard disk, hard drive or fixed disk[b] is a data storage device used for storing and retrieving digitalinformation using one or more rigid rapidly rotating disks (platters) coated with magnetic material. The platters are paired withmagnetic heads arranged on a moving actuator arm, which read and write data to the platter surfaces.[2] Data is accessed in arandom-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially. HDDs are a type of non-volatile memory, retaining stored data even when powered off.

Introduced by IBM in 1956,[3] 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 HDDs historically, though after extensive industry consolidation most current units are manufactured by Seagate, Toshiba, and Western Digital. As of 2016, HDD production (in bytes per year) is growing, although unit shipments and sales revenues are declining. The primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs), which have higher data-transfer rates, higher areal storage density, better reliability,[4] and much lower latency and access times.[5][6][7][8] While SSDs have higher cost per bit, SSDs are replacing HDDs where speed, power consumption, small size, and durability are important.[7][8]

The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion 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.

Hard disk drives were introduced in 1956 as data storage for an IBM real-time 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 medium-sized refrigerators and stored five million six-bit characters (3.75 megabytes)[9] on a stack of 50 disks.[16]

The IBM 350 RAMAC disk storage unit was superseded by the IBM 1301 disk storage unit,[17] which consisted of 50 platters, each about 1/8-inch thick and 24 inches in diameter.[18] Whereas the IBM 350 used two read/write heads, pneumatically actuated[16] 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.[19] Known as fixed-head or head-per-track disk drives they were very expensive and are no longer in production.[20]

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 14 inches (360 mm) 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 5 14 in (130 mm) 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/XTin 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. Many Macintosh computers made between 1986 and 1998 featured a SCSI port on the back, making external expansion simple. Older compact Macintosh computers did not have user-accessible hard drive bays (indeed, the Macintosh 128K, Macintosh 512K, and Macintosh Plusdid not feature a hard drive bay at all), so on those models external SCSI disks were the only reasonable option for expanding upon any internal storage.

The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013.[21]

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.

modern HDD records data by magnetizing a thin film of ferromagnetic material[f] on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding,[g] which determines how the data is represented by the magnetic transitions.

A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection.[23][24][25] For reference, a standard piece of copy paper is 0.07–0.18

The platters in contemporary HDDs are spun at speeds varying from 4,200 rpm in energy-efficient portable devices, to 15,000 rpm for high-performance servers.[28] The first HDDs spun at 1,200 rpm[3] and, for many years, 3,600 rpm was the norm.[29] As of December 2013, the platters in most consumer-grade HDDs spin at either 5,400 rpm or 7,200 rpm.[30]

Information is written to and read from a platter as it rotates past devices called read-and-write heads that are positioned to operate very close to the magnetic surface, with their flying heightoften in the range of tens of nanometers. The read-and-write head is used to detect and modify the magnetization of the material passing immediately under it.

In modern drives, there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor. Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track but modern drives (since the 1990s) use zone bit recording—increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects, thermally induced magnetic instability which is commonly known as the “superparamagnetic limit”. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.[31] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, first shipped in 2005,[32] and as of 2007 the technology was used in many HDDs.[33][34][35]

In 2004, a new concept was introduced to allow further increase of the data density in magnetic recording, using recording media consisting of coupled soft and hard magnetic layers. That so-called exchange spring media, also known as exchange coupled composite media, allows good writability due to the write-assist nature of the soft layer. However, the thermal stability is determined only by the hardest layer and not influenced by the soft layer.

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