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