| A hard disk (commonly known as a HDD (hard disk | | | | many small sub-micrometre-sized magnetic regions, |
| drive) or hard drive (HD) and formerly known as a | | | | each of which is used to encode a single binary unit |
| fixed disk) is a non-volatile storage device which | | | | of information. In today's hard disks each of these |
| stores digitally encoded data on rapidly rotating | | | | magnetic regions is composed of a few hundred |
| platters with magnetic surfaces. Strictly speaking, | | | | magnetic grains. Each magnetic region forms a |
| "drive" refers to a device that drives (removable) | | | | magnetic dipole which generates a highly localised |
| media, such as a tape drive or (floppy) disk drive, | | | | magnetic field nearby. The write head magnetizes a |
| while a hard disk contains fixed (non-removable) | | | | magnetic region by generating a strong local magnetic |
| media. However, in recent times, the hard disk has | | | | field nearby. Early hard disks used the same inductor |
| become more commonly known as the "hard drive." | | | | that was used to read the data as an electromagnet |
| Hard disks were originally developed for use with | | | | to create this field. Later versions of inductive heads |
| computers. In the 21st century, applications for hard | | | | included, metal in Gap (MIG) heads and thin film |
| disks have expanded beyond computers to include | | | | heads. In today's heads the read and write elements |
| digital video recorders, digital audio players, personal | | | | are separate but are in close proximity on the head |
| digital assistants, and digital cameras. In 2005 the first | | | | portion of an actuator arm. The read element is |
| mobile phones to include hard disks were introduced | | | | typically magneto-resistive while the write element is |
| by Samsung Group and Nokia. The need for | | | | typically thin-film inductive[2]. |
| large-scale, reliable storage, independent of a | | | | Hard disks have a mostly sealed enclosure that |
| particular device, led to the introduction of | | | | protects the disk internals from dust, condensation, |
| configurations such as RAID, hardware such as | | | | and other sources of contamination. The hard disk's |
| network attached storage (NAS) devices, and | | | | read-write heads fly on an air bearing which is a |
| systems such as storage area networks (SANs) for | | | | cushion of air only nanometers above the disk |
| efficient access to large volumes of data. | | | | surface. The disk surface and the disk's internal |
| Hard disks record data by magnetizing a magnetic | | | | environment must therefore be kept immaculate to |
| material in a pattern that represents the data. They | | | | prevent damage from fingerprints, hair, dust, smoke |
| read the data back by detecting the magnetization | | | | particles and such, given the sub-microscopic gap |
| of the material. A typical hard disk design consists of | | | | between the heads and disk. |
| a spindle which holds one or more flat circular disks | | | | Using rigid platters and sealing the unit allows much |
| called platters, onto which the data is recorded. The | | | | tighter tolerances than in a floppy disk drive. |
| platters are made from a non-magnetic material, | | | | Consequently, hard disk drives can store much more |
| usually glass or aluminum, and are coated with a thin | | | | data than floppy disk drives and access and transmit |
| layer of magnetic material. Older disks used iron(III) | | | | it faster. In 2007, a typical enterprise, i.e. workstation |
| oxide as the magnetic material, but current disks use | | | | hard disk might store between 160 GB and 750 GB |
| a cobalt-based alloy. | | | | of data (as of local US market by December 2006), |
| The platters are spun at very high speeds. | | | | rotate at 7,200 to 10,000 revolutions per minute |
| Information is written to a platter as it rotates past | | | | (RPM), and have a sequential media transfer rate of |
| mechanisms called read-and-write heads that fly very | | | | over 80 MB/s. The fastest enterprise hard disks spin |
| close over the magnetic surface. The read-and-write | | | | at 15,000 RPM, and can achieve sequential media |
| head is used to detect and modify the magnetization | | | | transfer speeds up to and beyond 110 MB/s.[3] |
| of the material immediately under it. There is one | | | | Mobile, i.e., Laptop hard disks, which are physically |
| head for each magnetic platter surface on the | | | | smaller than their desktop and enterprise |
| spindle, mounted on a common arm. An actuator arm | | | | counterparts, tend to be slower and have less |
| (or access arm) moves the heads on an arc (roughly | | | | capacity. In the 1990's, most spun at 4,200 RPM. In |
| radially) across the platters as they spin, allowing each | | | | 2007 a typical mobile hard disk spins at 5,400 RPM |
| head to access almost the entire surface of the | | | | and 7,200 RPM models are readily available for a slight |
| platter as it spins. | | | | price premium. |
| The magnetic surface of each platter is divided into | | | | |