Annualized Failure Rate (AFR) - A method of measuring failure rates or trends for a group of units at a site. The rates are based on the monthly total number of returned field failure units divided by the total cumulative installed base and multiplied by 12 (to annualize the failure rate).

AT Bus Attachment (ATA-4) - The interface defined by International Business Machines for the original AT disk controller. Western Digital designed the WD Caviar drives to be fully ATA-4 compatible.

Auto Defect Retirement - If defective sectors are found during a read or write, they are automatically mapped out and relocated.

Auto Park - Turning off the drive power causes the WD Caviar drives to move the read/write heads to a safe non-data landing zone and locks them in place.

Average Seek Time - The average time it takes for the read/write head to move to a specific block of data on the hard drive. The average seek time is computed by dividing the time it takes to complete a large number of random seeks by the number of seeks performed.

Block - A group of bytes handled, stored, and accessed as a logical data unit, such as an individual file record.

Buffer - A temporary data storage area that compensates for a difference in data transfer rates and/or data processing rates between sender and receiver.

Correctable Error - An error that can be overcome by the use of Error Detection and Correction schemes.

Customer Configuration Code (CCC) - The CCC is located on the product label attached to the drive. The CCC is revised only when there are drive form, fit, or function changes.

Data Synchronizer - An electronic circuit that produces a clock signal that is synchronous with the incoming data stream. The clock signal is then used to decode the data.

Data Transfer Rate - The rate that digital data is transferred from one point to another, expressed in bits per second or bytes per second.

• Data Transfer Rate to Disk: The internal disk transfer rate in Mbits per second.
• Data Transfer Rate from the Buffer to the Host: Based on the transfer of buffered data in MB per second.

Dedicated Landing Zone - A designated radial zone on the disk chosen to avoid contact with the data cylinders, where contact starting and stopping occur by design.

Defect Free - A term used to describe recording surfaces that have no detectable defects.

Defect Management - A general methodology of eliminating data errors on a recording surface by mapping out known bad areas of the media.

Disk Transfer Rate - Speed at which data transfers to and from the disk media (actual disk platter); a function of recording the frequency. Typical units are bits per second (BPS), or bytes per second. Modern hard disks have an increasing range of disk transfer rates from the inner diameter to the outer diameter of the disk. This is called a "zoned" recording technique.

ECC On-the-Fly - A hardware correction technique that corrects errors in the read buffer prior to host transfer without any performance penalties. These error corrections are invisible to the host system because they do not require assistance from the drive’s firmware.

Embedded Servo Control - The embedded servo control concept generates accurate feedback informationto the head position servo system without requiring a full data surface (which is required with a "dedicated" servo control method).

Enhanced IDE (EIDE) - Expands upon an industry accepted standard to meet the needs of improved processor speeds and bus technologies. It improves device support, provides greater than 528 MB capacity, increases data transfer rates, and provides support for non-hard drive peripherals (such as CD-ROMs).

Error Correction Code - A mathematical algorithm that can detect and correct errors in a data field by adding check bits to the original data.

Error Rate - The number of errors of a given type that occur when reading a specified number of bits.

FIT (Functional Integrity Testing)

A suite of tests Western Digital performs on all its drive products to ensure compatibility with different hosts, operating systems, application programs, and peripherals. This testing must be performed before the product can be released to manufacturing.

Flow Control

The ability of the drive to control the speed at which the host transfers data to or from the drive by using the IORDY signal. The host temporarily stops transferring data whenever the drive deasserts the IORDY signal. When the drive reasserts the IORDY signal, the host continues the data transfer.

Formatted Capacity - The actual capacity available to store data in a mass storage device. The formatted capacity is the gross capacity minus the capacity taken up by the overhead data required for formatting the media.

GB (Gigabyte) - Western Digital defines a gigabyte as 1,000,000,000 bytes.

Hard Error - An error that cannot be overcome by the error recovery process.

Hard Sectored - A technique that uses a digital signal to indicate the beginning of a sector on a track.

Host Transfer Rate - Speed at which the host computer can transfer data across the EIDE interface. Processor Input/Output (PIO) modes and Direct Memory Access (DMA) modes are defined in the ATA-4 industry specifications.

Index Pulse Signal - A digital pulse signal indicating the beginning of a disk revolution. An embedded servo pattern or other prerecorded information is present on the disk following index.

Landing Zone - The heads move to this location on the inner cylinders following a Park command. User data is not stored at this location.

Latency - The period of time that the read/write heads wait for the disk to rotate the data to an accessible position. For a disk rotating at 5400 RPM, the average latency is 5.5 milliseconds.

Logical Address - A storage location address that may or may not relate directly to a physical location. The logical address is usually used when requesting information from a controller. The controller performs a logical-to-physical address conversion and retrieves the data from a physical location in the storage device.

Logical Block Address - An alternative addressing methodology of identifying a given location on an EIDE drive that permits disk sizes greater than 528 MB.

Magnetoresistive Heads (MR Heads) - WD Caviar drives incorporate magnetoresistive heads to meet increasing capacity requirements. Read and write elements in the MR head are separate in order to optimize read and write capabilities. The read element, composed of a thin-film material whose resistance changes in the presence of a magnetic field, produces a strong signal, allowing significant increases in areal densities.

Mechanical Latencies - Include both seek time and rotational latency. Mechanical latencies are the main hindrance to higher performance in modern EIDE hard disks. The time delays of mechanical latencies are one hundred times higher than electronic (non-mechanical) latencies associated with the transferring of data. Therefore, reducing mechanical latencies (a lowering of seek time and rotational latency) should be the top consideration in improving hard drive performance.
See also: Seek Time, Rotational Latency.

MB (Megabyte) - Western Digital defines a megabyte as 1,000,000 bytes.

MTBF (Mean Time Between Failures) - Average time (expressed in hours) that a component works without failure. It is calculated by dividing the total number of operating hours observed by the total number of failures. Also, the length of time a user may reasonably expect a device or system to work before an incapacitating fault occurs.

MTTR - Mean Time to Repair.

PRML (Partial Response Maximum Likelihood) - A read channel using sampled data, active equalization and Veterbi detection to accurately retrieve the user data off the disk.

Rotational Latency - The amount of delay in obtaining information from a disk drive that can be attributed to the rotation of the disk. For a disk rotating at 5400 RPM, the average rotational latency is 5.5 milliseconds.
See also: Mechanical Latency.

RPM (Revolutions per Minute) - Rotational speed of the media (disk), also known as the spindle speed. Hard drives 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 of the disk transfer rate.

Sector - A 512-byte packet of data.

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.

Servo Burst - Provides positioning information to the actuator arm, usually positioned between sectors or at the end of a track.

Soft Error - A data error that can be overcome by the error recovery process.

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

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

Thermal Asperity - A thermal asperity is a baseline shift in the readback signal due to heating of the magnetoresistive stripe on the head as a result of physical contact with the disk or a particle.

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

Ultra ATA/66 - Data transmission protocol that delivers heightened data integrity to the EIDE interface through use of a 40-pin, 80-conductor cable. Ultra ATA/66 allows host computers to send and receive data at 66.6 MB/s, twice the 33.3 MB/s data transfer speed of Ultra ATA/33.

Write Cache - A feature in CacheFlow6 that posts "command complete" prior to completing the actual write.

Zoned Recording - Increases the data density on the outer tracks of the drive where most of the sectors are located. 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.

Information in this document was provided by the manufacturer.