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