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Hard Disk Drive

A hard disk drive is a non-volatile storage device for digital data. It has rotating rigid platters on a motor-driven spindle that is held in a metal case. Read/write heads float on a cushion of air about the platters and encode data magnetically. Hard disk manufacturers refer to disk capacity in standard powers of 1000. Hard disk drives were invented in 1956 by IBM and have been the dominant device for secondary storage of data on computers since due to their recording density, among other factors.
HDDs were originally developed to work with general purpose computers. However, the need for large-scale, reliable storage, independent of a particular device became prominent in the 1990s, which led to the introduction of embedded systems and storage area network systems that provide competent and consistent access to large volumes of data. Recently, HDD usage has expanded into consumer applications like cell phones and video game consoles.

HDDs record data by magnetizing ferromagnetic material directionally and reading the data back by detecting the magnetization of the material. A typical HDD has a spindle and platters that data are recorded onto. The platters are made from a non-magnetic material and are coated with a thin layer of magnetic material. The platters are spun at very high speeds, and information is written to it as it rotates past read-and-write heads that operate very closely over the magnetic surface. There is usually one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm moves the heads on an arc across the platters as they spin, allowing each head to access the surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor. The magnetic surface of each platter is divided into small magnetic regions that encode a single binary unit of information and is composed of a few hundred magnetic grains. Magnetic grains each form a single magnetic domain, which come together and form a magnetic dipole. This generates a highly localized magnetic field nearby. A write head magnetizes a region by producing a strong local magnetic field. Today, the read and write head elements are separate, but in close proximity, on the head portion of an actuator arm. The read element is usually magneto-resistive while the write element is classically thin-film inductive. HDD heads are kept separate from the platter surface by air that is extremely close to the platter and moves with it.

Capacities of HDDs are usually quoted with SI prefixes that increment by powers of 1000. Hard disks by contrast have no inherent binary size because capacity is determined by number of heads, tracks and sectors. For example, a one terabyte disk drive would be expected to hold around 1 trillion bytes or 1000 gigabytes. The capacity of an HDD can be found by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector.

A typical 7200 rpm desktop hard drive has sustained a disk to buffer data transfer rate of about 70 megabytes per second. This rate is dependent on the track location, so it will be higher for data on the outer tracks and lower toward the inner tracks. Data transfer rate can be measured by writing a large file to disk using exclusive file generator tools and then reading back the file. Transfer rate can be influenced by the layout of the files as well as the file system fragmentation. Seek time for non-sequential data ranges from 3 ms for high-end server drives to 15 ms for mobile drives. Most common desktop drives are about 9 ms. Power consumption has become increasingly important in hard drives and computer systems. Due to the increase in data center machine density, problems with delivering sufficient power to devices have arose. Similar problems are common for large companies with thousands of desktop computers. Smaller form factor drives often use less power than larger drives. Shock resistance is important, especially for mobile devices. Some laptops have active hard drive protection that parks the disk heads mid-air if the machine is dropped with the hopes of protecting it. Another form of protection is a program called S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology), which measures drive characteristics such as operating temperature and spin-up time. Certain trends and sudden changes in these characteristics are commonly associated with the chance of drive failure and data loss. Unfortunately, not all data loss is predictable. Normal use can eventually lead to a breakdown in the device, which makes it essential for the user to regularly back up the data onto a separate storage device. Eventually all mechanical hard disk drives fail, so to prevent loss of data, it needs to be backed up regularly.

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Hard Disk Drive


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