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