Hard Drive Terms

ATA - Stands for Advanced Technology Attachment. It is the official name for IDE.

Cache - A high speed RAM buffer between the CPU and the hard drive. When data is read from the hard drive, a copy is saved in the cache. If that same data is read again, it will be retrieved from the cache and returned quicker than if read from the hard drive again.

Cluster - A unit of storage allocation. The cluster size is defined during formatting and varies depending on the size of the hard drive and the file system. It is usually from 4K to 32K in size. A file stored on a hard drive uses at least one cluster.

DMA - Stands for Direct Memory Access. It is an efficient process of transferring data directly to and from main memory without involving the CPU.

EIDE - Stands for Enhanced Integrated Drive Electronics. It is an enhanced version of the IDE interface. The improvements include the ability to address larger hard drives, faster access through DMA, and support for additional drives such as the CD-ROM.

Formatted Capacity - This is the actual size of the hard drive minus the overhead data needed for formatting the media.

Form Factor - This defines the physical and external dimensions of a device. For hard drives, this term is associated with the width of the drive. A notebook hard drive has a 2.5-inch form factor and a regular desktop hard drive usually has a 3.5-inch form factor.

IDE - Stands for Integrated Drive Electronics. This is a 16-bit drive interface that has the controller electronics incorporated into the drive itself rather than on a separate card. Modern hard drives today use the EIDE interface.

Latency - This is the amount of time the read/write heads wait for the disk to rotate to the correct position to access data. The faster the RPM of the hard drive, the lower the latency period. The average latency is calculated by taking half the time it takes for a full rotation. The average latency for a 4200 RPM hard drive is 7.1ms. For 5400 RPM, the average latency is 5.6ms. For 7200 RPM, the average latency is 4.2ms.

MTBF - Stands for Mean Time Between Failure. It is the average number of hours that a hard drive will work before failure. It is determined by testing a large number of the same hard drive simultaneously and seeing how many hours pass before a failure occurs.

PIO - Stands for Programmed Input/Output. It is a method of moving data by sending control data through the CPU. There are different modes of PIO which specify the transfer rate. They go from mode 0 at 3.3 MBps all the way up to mode 4 at 16.6 MBps. A better alternative to PIO is DMA which most newer devices support.

RPM - Stands for Revolutions Per Minute. This is simply the rotational speed of the disks in a hard drive.

SCSI - Stands for Small Computer System Interface. This is a communication interface developed by Apple Computer. It allows several devices to be connected in a daisy-chain fashion. Rather than having a separate card for each device, one controller card is used for all of them.

Platter - This is the actual disk that is mounted inside a hard drive. Usually a hard drive consists of multiple platters to provide more storage capacity. The platter can be double sided and use both surfaces to store data.

RAID - Stands for Redundant Arrays of Independent Disks. This is where multiple hard drives are grouped and seen by the OS as one logical hard drive. RAID provides greater data integrity through redundancy and greater performance.

Read/Write Head - This is a metal pole consisting of an electromagnetic coil that is used to read and write magnetic patterns on the disk. There is a read/write head for each data surface.

SATA - Stands for Serial Advanced Technology Attachment. This is an interface for connecting hard drives to a computer. Unlike IDE which uses parallel signaling, SATA uses serial signaling technology. Because of this the SATA cables are thinner than the ribbon cables used by IDE hard drives. SATA cables can also be longer allowing you to connect to more distant devices without fear of signal interference. There is also more room to grow with data transfer speeds starting at 150 MB/s.

Sector - The smallest unit of storage on a hard drive. A sector on a hard drive consists of a 512-byte packet of data.

Seek Time - This is a measure in milliseconds of how fast the hard drive can move its read/write heads to a desired location.

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

Ultra ATA or UATA - A high speed data transmission protocol consisting of various data transfer speeds of 33, 66, 100 and 133 MB/s. This term is used interchangeably with Ultra DMA.

Ultra DMA or UDMA - This is a protocol for transferring data between a hard drive through the bus to the computer's memory.

Unformatted Capacity - This is the total size of a hard drive including overhead space.