Hard Disk Drive Terminology

Glossary Hard Disk Drive Terminology: From Letter A to Letter Z. If you enjoy it, just add it to your favorite!

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Zoned Recording
Increases the number of sectors on the outer tracks of the drive since the circumference of the outside tracks is greater. This type of recording affords more disk capacity because there can be more sectors on the larger outer tracks than would be possible if the number of sectors per track were constant for the whole drive.

Year 2000 Problem, or Y2K
The risk that computers will be disabled or perform incorrectly if they read the two-digit abbreviation “00” to mean the year 1900 instead of the year 2000, as intended. The problem is said to have originated when early computer programmers abbreviated year dates to save storage space that was scarce and costly at the time. Western Digital hard drives are designed to be “Year 2000-compliant,” and the company’ suppliers are required to demonstrate that their WD-linked communications and products are compliant as well.

WD Caviar®
The trade name for Western Digital’s line of hard drives for desktop personal computers.

WD Enterprise
The trade name for Western Digital’s line of enterprise hard drives.

Winchester Disk
Former code name for an early IBM hard disk model, sometimes still used to refer to the technology and design of most traditional hard drives.

Windows
Microsoft’s series of operating systems for personal computers. Currently popular versions are Windows 95 and Windows 98.

Word
Two bytes that are processed together in a single operation.

Workstation
A personal computer with exceptional capacity and performance capabilities for use mainly in engineering, design and audiovisual applications demanding immediate access to data and the ability to manipulate it in technically sophisticated ways.

Write
The recording of flux reversals onto the magnetic surface of a disk.

Write Cache
High speed RAM used to buffer data transfer from the host to the hard drive.

Write Verify
Immediately after writing data to the disk, a drive with the Write Verify feature will verify that it can read the data it just wrote to the disk to ensure that it will be able to retrieve it later. If the drive is unable to read the data, it writes it to another area of the disk, where it attempts to write verify it again.

Value-added Reseller (VAR)
Often referred to as a “Var” or “Reseller.” The independent professionals who market computer systems and components and who add value by performing assembly, upgrades, training and other services.

Viterbi Detection
An algorithm used in read channel technology that detects an entire sequence of data bits at a time and determines the most likely sequence of data bits by comparing actual sequence of data bit samples with sequences of possible data bit sample to accurately detect that data written to disk.

Voice Coil
An actuator motor; the force of the magnetic rotary voice coil produces a movement of the head that is proportionate to the force exerted by the coil.

Ultra SCSI
Provides 20 MB/s transfers over an 8-bit bus or 40 MB/s transfers over a 16-bit Wide SCSI bus. Also known as Fast-20 SCSI, this feature is most commonly found in SCSI-3 drives.

Ultra DMA/33
A high-speed host data transfer feature that transfers data at 33.3 MB per second.

Un-correctable Error
An error that cannot be overcome using Error Detection and Correction.

Unformatted Capacity
The total number of usable bytes on a disk, including the space that is required to record location, boundary definitions, and servo data. (See also formatted capacity.)

Unrecoverable Error
A read error that cannot be overcome by an ECC scheme or by rereading the data when host retries are enabled.

Untagged Queuing
The ability of the drive to receive a maximum of one I/O process from each initiator.

Upgrade
In hard drives, the replacement of a hard drive with one offering greatercapacity or performance, or both.

Tagged Queuing
The ability of the drive to receive multiple I/O processes from each initiator.

Task File
The set of I/O Host Interface Registers used to transfer status, commands, and data between the host and the drive for the EIDE interface.

Thin Client Architecture
A computer system in which data is stored centrally, with only limited storage capacity at the various points of use.

Thin Film
A type of coating deposited on a flat surface through a photolithographic process. Thin film is used on disk platters and read/write heads, as well as on the write element of MR heads.

Thin-Film Inductive Head (TFI)
A head technology that uses a thin-film inductive element to read and write data bits on the magnetic surface of the disk.

Time-to-Capacity
Getting to market first with the next highest capacity hard drive.

Time-to-Market
The time it takes to bring a product from concept to market. Generally first-to-market is the desired time-to-market goal.

Time-to-Quality
The time required to bring a new product to market with the best possible level of quality and reliability.

Time-to-Volume
The time required to begin producing a new product in sufficiently high volume to fill commercial requirements.

TPI (Tracks per inch)
The number of tracks written within each inch of the disk’s surfaces, used to measure how closely the tracks are packed on a disk surface. Also known as track density.

Track
A concentric magnetic circle pattern on a disk surface used for storing and reading data.

Track-to-track Seek Time
The time that elapses when the read/write heads move from one track to an adjacent track.

Transfer Rate
The rate at which the hard drive sends and receives data from the controller. Processing, head switches, and seeks are all figured into the transfer rate in order to accurately portray drive performance. The burst mode transfer rate is separate from transfer rate, as it refers only to the transfer of data into RAM.

Translating BIOS
A system BIOS that allows access to EIDE drives larger than 528 MB.

SCA-2
SCA-2 (Single Connector Attach) interface incorporates a grounding contact, blindmate connector, direct plug misalignment tolerance, ESD protection, hot swap capability, and backplane connector options for SCSI devices. SCA-2 is commonly called the 80-pin SCSI connector.

Small Computer System Interface (SCSI)
An interface between a computer and peripheral controllers. Commonly used in enterprise computing and in Apple Macintosh systems. Usually pronounced as “scuzzy.” The equivalent interface system in most personal computers is Enhanced Integrated Drive Electronics, usually called EIDE.

SCSI Configure Automatically (SCAM)
Allows users to attach SCSI devices without worrying about configuration options.

SCSI-1
The Small Computer System Interface (ANSI document X3.131-1986).

SCSI-2
The Small Computer System Interface (ANSI document X3.131-1994).

SCSI-3
The ANSI X3T10 Working Documents (under development).

SCSI device
A host computer adapter, a peripheral controller, or an intelligent peripheral that can be attached to a SCSI bus.

Sector
A 512-byte packet of data in EIDE and SCSI hard drives. This is the smallest amount of data that can be read or written to the drive from the host interface. On Macintosh and Unix drives, sectors are usually grouped into blocks or logical blocks that function as the smallest data unit permitted. Since these blocks are often defined as a single sector, the terms block and sector are sometimes used interchangeably in this context. (Note: The meaning of the term block in connection with the physical configuration of the disk is different from its meaning at the
system level. (See also block and cluster.)

Sector Slipping
A technique used to push-down defective sector sites during a format or reassignment operation to maintain sequential order of the data. Spares are located throughout the disk for this purpose.

Seek
The movement of a set of read/write heads to a desired location. The actuator moves the heads to the cylinder containing the track and sector where the data is stored.

Seek Time
A measure (in milliseconds) of how fast the hard drive can move its read/write heads to a desired location.

Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T.)
A technology to assist the user in preventing possible system down time due to hard drive failure by attempting to predict imminent hard drive failure before it occurs.

Sequential Access
The reading or writing of data in a sequential order as opposed to random access. Magnetic tape drives store data in sequential blocks.

Serial Storage Architecture (SAA)
The general name given to a set of standards being developed by an ANSI-approved X3 group. The set of standards defines a new serial interface that provides a flexible addressing scheme.

Server
A computer used primarily to store data, providing access to shared resources. Usually contains a network operating system.

Servo Burst
Provides positioning information to the actuator arm, found at equal intervals on each disk surface (embedded servo) or on an entire surface (dedicated servo).

Single-ended SCSI
The standard electrical interface for SCSI. Single-ended means an interface with one signal and one corresponding ground line for each SCSI signal. Used primarily in applications requiring cable lengths under 19 feet (6 meters).

Slave
The second drive in a dual drive combination. Soft Error An error that does not repeat when the same location is re-read.

Soft Sectored
A technique that allows the controller to determine the beginning of a sector by reading the format information from the disk.

SPC
SCSI Primary Commands.

Spindle
The center, rod-like axle on which the disks are mounted.

Spindle Motor
The motor that rotates the spindle and ultimately the disks.

Spindle Speed
See RPM.

Spindle Synchronization
A feature that causes SCSI hard drives in multiple-drive systems to rotate to the same address location at the same time.

Spreadsheet
The display of data in a form suitable for comparisons, as is used in accounting and other applications; also, computer programs for these uses.

Storage Capacity
The amount of data that can be stored on a hard drive.

Sub-1000 PC
The series of personal computers being designed for sale at prices at or below $1,000 each.

Subsystem
A secondary or component part of a system, as a hard drive is a subsystem of a personal computer.

Surface
The top or the bottom side of a platter coated with a magnetic material required to record data. A platter may use one or both surfaces to store data.

Synchronous Transmission
Transmission in which the sending and receiving devices operate continuously at the same frequency and are held in a desired phase relationship by a correction device.

System Files
The files needed to run an operating system.

System Integrator
An independent professional who specifies and provides the necessary combinations of hardware and software in response to an end user’s needs.

Radial Path
The straight-line path from the center of the disk to the outer edge of the disk.

Random Access Memory (RAM)
Memory that allows any storage location to be accessed randomly, as opposed to tape drives, which are sequential access devices.

Read Channel
Performs the data encoding and conversions the drive needs to write computer generated information onto a magnetic medium and then read that information back with a high degree of accuracy.

Read Verify
A data accuracy check performed by having the disk read data to the controller, which in turn checks for errors but does not pass the data on to the system.

Read/Write Head
See Head.

Recoverable Error
A read error that the drive can correct by ECC recovery or by re-reading the data.

Redundant Arrays of Independent Disks (RAID)
Groupings of hard drives in a single system to provide greater performance and data integrity.

R & D
Research and development.

RLL (Run Length Limited)
An encoding scheme used during write operations to facilitate data readback.

ROM (Read Only Memory)
Integrated circuit memory chip containing programs and data that the computer or host can read but cannot modify. The computer can read instructions out of ROM, but cannot store data in ROM.

Rotational Latency
The amount of delay in obtaining information from a disk due to the rotation of the disk. For a disk rotating at 5200 RPM, the average rotational latency is 5.8 milliseconds. See also Mechanical Latency.

RPM (Revolutions per Minute)
Rotational speed of the media (disk), also known as the spindle speed. Hard drives typically spin at one constant speed. The slower the RPM, the higher the mechanical latencies. Disk RPM is a critical component of hard drive performance because it directly impacts the rotational latency.

Qualification
The process by which sample components are tested for their compatibility and utility as parts of a system. Western Digital OEM customers qualify WD hard drives for use in their computers.

Queue
A first-in-first-out (FIFO) data structure used to sequence multiple demands for a resource such as a printer, processor, or communications channel. The host adds objects to the end of the queue and takes them off the front.