Hard disk drive

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk or fixed disk drive, is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media.
Strictly speaking, an HDD is a rigid-disk drive, although it is probably never referred to as such. By way of comparison, a so-called "floppy" drive (more formally, a diskette drive) has a disc that is flexible. Originally, the term "hard" was temporary slang, substituting "hard" for "rigid", before these drives had an established and universally-agreed-upon name. Some time ago, IBM's internal company term for an HDD was "file".
HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia. The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Technology
HDDs record data by magnetizing ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.
The platters are spun at very high speeds (details follow). Information is written to a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. 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 older designs) a stepper motor. Stepper motors were outside the head-disk chamber, and preceded voice-coil drives. The latter, for a while, had a structure similar to that of a loudspeaker; the coil and heads moved in a straight line, along a radius of the platters. The present-day structure differs in several respects from that of the earlier voice-coil drives, but the same interaction between the coil and magnetic field still applies, and the term is still used.
Older drives read the data on the platter by sensing the rate of change of the magnetism in the head; these heads had small coils, and worked (in principle) much like magnetic-tape playback heads, although not in contact with the recording surface. As data density increased, read heads using magnetoresistance (MR) came into use; the electrical resistance of the head changed according to the strength of the magnetism from the platter. Later development made use of spintronics; in these heads, the magnetoresistive effect was comparatively huge, compared to that of earlier types, and was dubbed "giant" magnetoresistance (GMR) referreing to the degree of effect, not the physical size; the heads themselves are extremely tiny, too small to see without a microscope. GMR read heads are now commonplace.

Capacity and access speed
Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster. As of January 2008:
A typical desktop HDD, might store between 120 and 300 GBs of data (based on US market data), rotate at 7,200 revolutions per minute (RPM) and have a media transfer rate of 1 Gbit/s or higher.
The highest capacity HDDs are 1
The fastest “enterprise” HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s. Drives running at 10,000 or 15,000 rpm use smaller platters because of air drag and therefore generally have lower capacity than the highest capacity desktop drives.
Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. A typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium. Because of the smaller disks, mobile HDDs generally have lower capacity than the highest capacity desktop drives.
The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as digital video recorders and digital audio players. In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search and email (Google, Yahoo!, etc.).
The main way to decrease access time is to increase rotational speed, while the main way to increase throughput and storage capacity is to increase areal density. A vice president of Seagate Technology projects a future growth in disk density of 40% per year. Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.
As of 2006, some disk drives use perpendicular recording technology to increase recording density and throughput.
The first 3.5" HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space.Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.