Glossary

Access

(v) Read, write, or update information on some storage medium, such as a disk. (n) The operation of reading, writing, or updating stored information.

Access time

The interval between the time a request for data is made by the system and the time the data is available from the drive. Access time includes the actual seek time, rotational latency, and command processing overhead time. See also average seek time, rotational latency, and overhead.

Actuator

The internal mechanism that moves the read/write head to the proper track. The Quantum actuator consists of a rotary voice coil and the head mounting arms. One end of each head mounting arm attaches to the rotor with the read/write heads attached at the opposite end of each arm. As current is applied to the rotor, it rotates, positioning the heads over the desired cylinder on the media. Also known as the rotary actuator or positioner.

Allocation

The process of assigning particular areas of the disk to particular data or instructions. See also allocation unit.

Allocation unit

An allocation unit is a group of sectors on the disk that can be reserved for the use of a particular file. On hard disks for small computer systems, the allocation unit is usually in the form of a sector, block, or cluster.

Average seek time

The average time it takes for the read/write head to move to a specific location. To compute the average seek time, divide the time it takes to complete a large number of random seeks by the number of seeks performed.

Backup

A copy of a file, directory, or volume on a separate storage device from the original, for the purpose of retrieval in case the original is accidentally erased, damaged, or destroyed.

Bad block

A block (usually the size of a sector) that cannot reliably hold data because of a media flaw or damaged format markings.

Bad track table

A label affixed to the casing of a hard disk drive that tells which tracks are flawed and cannot hold data. The list is typed into the low-level formatting program when the drive is being installed. Quantum's built-in defect-management protections compensate for these flaws automatically.

Bezel

A plastic panel that extends the face of a drive so that it covers a computer's drive bay opening. The bezel usually contains a drive-activity LED. Also known as the faceplate.

Bit

Abbreviation for binary digit. A binary digit may have one of two values - 1 or 0. This contrasts with a decimal digit, which may have a value from 0 to 9. A bit is one of the logic 1 or logic 0 binary settings that make up a byte of data. See also byte.

Block

A sector or group of sectors. By default, a sector of data consists of 512 bytes.

BPI

Acronym for bits per inch. A measure of how densely information is packed on a storage medium. See flux density.

Buffer

An area of RAM reserved for temporary storage of data that is waiting to be sent to a device. The data is usually on its way to or from the hard disk drive or some other peripheral device.

Bus

The part of a chip, circuit board, or interface designed to send and receive data.

Byte

The basic unit of computer memory, large enough to hold one character of alphanumeric data. Comprised of eight bits. See also bit.

Cache

Specialized RAM used to optimize data transfers between system elements with different performance characteristics, e.g., disk to main memory or main memory to CPU.

Capacity

The amount of information that can be stored on a hard disk drive. The data is stored in bytes, and capacity is usually expressed in megabytes.

Clean room

An environmentally controlled dust-free assembly or repair facility in which hard disk drives are assembled or can be opened for internal servicing.

Cluster

A group of sectors on a hard disk drive that is addressed as one logical unit by the operating system.

Controller

The chip or circuit that translates computer data and commands into a form suitable for use by the hard drive. Also known as the disk controller.

Controller card

An adapter containing the control electronics for one or more hard disks. Usually installed in a slot in the computer.

CPU

Acronym for central precessing unit. The microprocessor chip that performs the bulk of data processing in a computer.

CRC

Acronym for cyclic redundancy check. An error detection code that is recorded within each sector and is used to see whether parts of a string of data are missing or erroneous.

Cylinder

When disks are placed directly above one another along the shaft, the circular, vertical "slice" consisting of all the tracks located in a particular position.

Data separator

On a hard disk drive that stores data and timing information in an encoded form, the circuit that extracts the data from the combined data and clock signal.

Dedicated servo

A positioning mechanism using a dedicated surface of the disk that contains timing and positioning information only, as compared to surfaces that also are used for data.

Defect management

A technique ensuring long-term data integrity. Defect management consists of scanning disk drives both at the factory and during regular use, deallocating defective sectors before purchase and compensating for new defective sectors afterward.

Disk

In general, any circular-shaped data-storage medium that stores data on the flat surface of the platter. The most common type of disk is the magnetic disk, which stores data as magnetic patterns in a metal coating. Magnetic disks come in two forms: floppy and hard. Optical recording is a newer disk technology that gives higher capacity storage but at slower access times. DMA - Acronym for direct memory access. A process by which data moves directly between a disk drive (or other device) and system memory without passing through the CPU, thus allowing the system to continue processing other tasks while the new data is being retrieved.

Drive geometry

The functional dimensions of a drive in terms of the number of heads, cylinders, and sectors per track.

ECC

Acronym for Error Correction Code. The incorporation of extra parity bits in transmitted data in order to detect errors that can be corrected by the controller.

Embedded servo

A timing or positioning signal included on the disk's surface that also stores data, allowing the actuator to fine tune the position of the read/write heads.

Encoding

The protocol by which particular data patterns are changed prior to being written on the disk surface as a pattern of On and Off or 1 and 0 signals.

EPROM

Acronym for erasable programmable read only memory. An integrated circuit memory chip that can store programs and data in a non-volatile state. These de-vices can be erased by ultraviolet light and reprogrammed with new data. External drive - A drive mounted in an enclosure, separate from the computer system enclosure, with its own power supply and fan, and connected to the system by a cable.

Faceplate

See bezel.

FCI

Acronym for flux changes per inch. See also BPI.

File server

A computer that provides network stations with controlled access to shareable resources. The network operating system is loaded on the file server, and most shareable devices (disk subsystems, printers) are attached to it. The file server controls system security and monitors station-to-station communications. A dedicated file server can be used only as a file server while it is on the network. A non-dedicated file server can be used simultaneously as a file server and a workstation.

Firmware

Permanent instructions and data programmed directly into the circuitry of read-only memory for controlling the operation of the computer or disk drive. Distinct from software, which is stored in read/write memory and can be altered.

Flux density

The number of magnetic field patterns that can be stored on a given area of disk surface, used as a measure of data density. The number is usually stated as flux changes per inch (FCI), with typical values in the tens of thousands.

Flying height

The distance between the read/write head and the disk surface, made up of a cushion of air that keeps the head from contacting the media. Smaller flying heights permit denser data storage but require more precise mechanical designs.

Format

A magnetic track pattern that specifies the locations of the tracks and sectors. This information must exist on a disk before it can store any user data. Formatting erases any previously stored data.

Formatted capacity

The amount of room left to store data on a disk after writing the sector headers, boundary definitions, and timing information during a format operation. The size of a Quantum drive always is expressed in formatted capacity, accurately reflecting the usable space available.

Form factor

The industry standard that defines the physical, external dimensions of a particular device. Quantum hard disk drives use 3.5-inch or 2.5-inch form factors.

Gigabyte (GB)

A unit of measure consisting of one billion bytes (one thousand megabytes). Guide rails - Plastic or metal strips attached to the sides of a hard disk drive mounted in an IBM AT and compatible computers so that the drive easily slides into place.

Half height

Standard drive size equivalent to half the vertical space of a 5.25-inch drive.

Hard disk

A type of storage medium that retains data as magnetic patterns on a rigid disk, usually made of a magnetic thin film deposited on an aluminum platter. Because hard disks spin more rapidly than floppy disks, and the head flies closer to the disk, hard disks can transfer data faster and store more in the same volume.

Hard error

A data error that persists when the disk is reread, usually caused by defects in the physical surface.

HDA

Acronym for head disk assembly, typically a sealed unit.

Head

The tiny electromagnetic coil and metal pole used to create and read back the magnetic pat-terns on the disk. Also known as the read/write head.

Head crash

Damage to a read/ write head and magnetic media, usually caused by sudden contact of the heads with the disk surface. Head crash also can be caused by dust and other contamination inside the HDA. High-capacity drive - By industry conventions, typically a drive of 500 megabytes or more.

High-level formatting

Formatting performed by the operating system's format program (for example, the DOS FORMAT pro-gram). Among other things, the formatting program creates the root directory, file allocation tables, and other basic configurations. See also low-level formatting.

Home

Reference position track for recalibration of the actuator, usually the outer track (Track 0).

Host adapter

A plug-in board or circuitry on the motherboard that acts as the interface between a computer system bus and the disk drive. Typically used with embedded controller drives such as SCSI and IDE.

Initialize

See low-level formatting.

Interface

A hardware or software protocol, contained in the electronics of the disk controller and disk drive, that manages the exchange of data between the drive and computer. The most common interfaces for small computer systems are AT (IDE) and SCSI.

Interleave

To intersperse the sectors on a track. See also inter-leave factor.

Interleave factor

The number of sectors that pass beneath the read/ write heads before the next sector arrives. For example, a 3:1 interleave factor means that the heads read a sector, then let two pass by before reading another, requiring three full revolutions of the disk to access the complete data track. Quantum drives have an interleave of 1:1, allowing the system to access a full track of data in a single revolution. See also interleave.

Internal drive

A drive mounted inside one of a computer's drive bays (or a hard disk on a card, which is installed in one of the computer's slots).

Jumper

A tiny connector box that slips over two pins that protrude from a circuit board. The jumper can be moved to change electrical connectors. When in place, the jumper connects the pins electrically. Some board manufacturers use dual in-line package (DIP) switches instead of jumpers.

Kilobyte (KB)

A unit of measure consisting of 1,024 bytes.

Landing zone

A non-data area on the disk's inner cylinder where the heads can rest when the power is off.

Latency

The time during which the read/write heads wait for data to rotate into position after the controller starts looking for a particular data track. If a disk rotates at 3,600 rpm, the maximum latency time is 16.4 milliseconds, and the average latency time is 8.2 milliseconds. Also known as rotational latency.

Look ahead

The technique of buffering data into cache RAM by reading subsequent blocks in advance to anticipate the next request for data. The look ahead technique speeds up disk access of sequential blocks of data.

Low-level formatting

The process of creating sectors on the disk surface so that the operating system can access the required areas for generating the file structure. Quantum drives are shipped with the low-level formatting already completed. Also known as initialization.

Low profile

Describes drives built to the 3.5-inch form factor, which are only 1-inch high. The standard form factor drives are 1.625-inches high.