Glossary of PC terms: H

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Half height	One of the standard drive sizes equivalent to half the vertical dimension of a 5.25-inch drive.
Hard disk	Storage medium that stores data in the form of magnetic patterns on a rigid disk.  Modern hard disks are usually made of several thin films deposited on both sides of the aluminum, glass, etc. substrate. Hard disks are much more rigid than floppy disks, spin much faster than floppy disks, and can transfer data faster and store more in the same volume.
Hard error	A data error that does not go away with type (unlike the soft error), and is usually caused by defects in the physical structure of the disk.
Head (magnetic, read/write)	A component of storage systems (hard drives, tapes, CDs, floppy drives, etc.) used for data reading (read head) and writing (write head). Can operate on magnetic, magneto-optical, optical, or other principles.
Head crash	General term used to describe a catastrophic phenomenon which causes mechanical damage to the disks and/or head, and may lead to the permanent damage and unrecoverable loss of data. See also Data loss / recovery.
Head material	Today, magnetic heads consist of multiple layers of metal alloys and insulating materials. An inductive thin-film head has the "simplest" design, while a GMR (giant magneto-resistive) head has the most complex design.
Head Parking	Since the disk and the slider in magnetic hard drives are extremely smooth, strong adhesive forces may prevent disks from rotating (during "power-on" cycle) if the slider is landed on the disk surface.  To prevent this phenomenon, modern hard disks have a special landing zone - a narrow area close to the disk center which has been textured using a laser.  A spiral of tiny laser bumps is created, which increases a disk's roughness, decreases adhesion, and allows slider landing and take-off from the landing zone.   Another method of head parking is using a load-unload ramp fixed at the outer edge of the disk.  When the suspension is moved beyond the disk area, it slides onto the ramp thus parking the head.  Both landing on the landing zone and parking on the ramp increase the drive's non-operational shock resistance and prevent accidental damage during transportation.
Head landing / takeoff	A procedure performed during powering down and powering up CSS-type drives (most of today's desktop drives, many of hard drives for servers and laptops).
Head suspension	An elastic beam-like element of the magnetic drive design with the slider (and magnetic element) at one end and attached to the VCA (see Actuator; VCA) arm with the other end. Elastic deformation of the suspension creates the pre-load force needed for successful performance of the system.
Head-disk interface	A contact between a fast-spinning disk and a flying or sliding slider.  It is a source of many durability problems for magnetic drives - head crash is one example.
High-level formatting	Formatting is performed by the operating system's format program (for example, the DOS FORMAT program) that creates the root directory, file allocation tables, and other basic configurations. See also low-level formatting.
Host adapter	A plug-in board or circuitry on the motherboard acting as the interface between a computer system bus and the drive.  Typically used with embedded controller drives, such as SCSI and IDE.
Hot Plug (Hot Swap)	Procedure involving the plugging in or removal of a drive into a system with the power turned on.
HVD (High Voltage Differential)	HVD refers to SCSI-2 and SCSI-3 differential signals with a maximum logic voltage of 5V.  See also LVD.

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