In magnetic disk drives, it is the time for the access arm to reach the desired track and the delay for the rotation of the disk to bring the required sector under the read-write mechanism.

A gigabyte (derived from the SI prefix giga-) is a unit of information or computer storage equal to one billion bytes. It is commonly abbreviated GB in writing (not to be confused with GB, which is used for gigabit) and gig in writing or speech.

Usually, these devices connect to the computer through an Integrated Drive Electronics (IDE) interface. Essentially, an IDE interface is a standard way for a storage device to connect to a computer. IDE is actually not the true technical name for the interface standard. The original name, AT Attachment (ATA), signified that the interface was initially developed for the IBM AT computer. IDE was created as a way to standardize the use of hard drives in computers.

The basic concept behind IDE is that the hard drive and the controller should be combined. The controller is a small circuit board with chips that provide guidance as to exactly how the hard drive stores and accesses data. Most controllers also include some memory that acts as a buffer to enhance hard drive performance.

Initially SATA was designed as an internal or inside-the-box interface technology, bringing improved performance and new features to internal PC or consumer storage. Designers quickly realized the new interface could reliably be expanded outside the PC, bringing the same performance and features to external storage needs instead of relying on USB or FireWire (IEEE 1394) interfaces. Called external SATA or eSATA, the SATA devices can be plugged by shielded cable lengths up to two meters outside the PC. SATA is now out of the box as an external standard, with specifically defined cables, connectors, and signal requirements released as new standards in mid-2004. eSATA provides more performance than existing solutions and is hot pluggable.

Key benefits of eSATA:

• Up to six times faster than contemporary external storage solutions: USB 2.0 and Firewire (IEEE 1394)
• Robust and user friendly external connection
• High performance, cost effective expansion storage
• Up to 2-m shielded cables and connectors
• Applications of eSATA include External Direct Attached Storage for notebooks, desktop, consumer electronics and entry servers.
   

Many existing external hard drives use USB and/or FireWire. These interfaces are not nearly as fast as SATA when compared using peak values, and can theoretically compromise drive performance; although in actual tests, all standards have shown transfer rates well below their peak value, as well as differences among platforms (i.e. Mac versus Windows).

USB and FireWire external drives are ATA drives with a bridge chip that translates from the ATA protocol to USB or FireWire protocol used for the connection. These interfaces require encapsulation or conversion of the transmit data and then de-capsulation after the data is received. This protocol overhead reduces the efficiency of these host buses, increases the host CPU utilization or requires a special chip to off-load the host. The eSATA results in no protocol overhead issues unlike with USB or IEEE 1394. This ability is perfect for using an array of drives with performance striping behind the eSATA host port.

The typical cable length is two meters (six feet). The compliance is defined in the SATA II: Electrical Specification, as the Gen1m and Gen2m specifications for 1.5 Gb/s and 3.0 Gb/s respectively. Currently, most PC motherboards do not have an eSATA connector. eSATA may be enabled through the addition of an eSATA host bus adapter (HBA) or bracket connector for desktop systems or with a Cardbus or ExpressCard for notebooks. New motherboards introduced in 2005 may start to incorporate eSATA connectors directly.

Note: Prior to the final specification for eSATA, there were a number of products designed for external connections of SATA drives. Some of these use the internal SATA connector or even connectors designed for other interface specifications, such as IEEE 1394. These products are not eSATA compliant.

In computing, a redundant array of independent disks, also known as redundant array of inexpensive disks (commonly abbreviated RAID) is a system which uses multiple hard drives to share or replicate data among the drives. Depending on the version chosen, the benefit of RAID is one or more of increased data integrity, fault-tolerance, throughput or capacity compared to single drives. In its original implementations (in which it was an abbreviation for "redundant array of inexpensive disks"), its key advantage was the ability to combine multiple low-cost devices using older technology into an array that offered greater capacity, reliability, speed, or a combination of these things, than was affordably available in a single device using the newest technology.

At the very simplest level, RAID combines multiple hard drives into a single logical unit. Thus, instead of seeing several different hard drives, the operating system sees only one. RAID is typically used on server computers, and is usually (but not necessarily) implemented with identically-sized disk drives. With decreases in hard drive prices and wider availability of RAID options built into motherboard chipsets, RAID is also being found and offered as an option in more advanced user computers. This is especially true in computers dedicated to storage-intensive tasks, such as video and audio editing.

The original RAID specification suggested a number of prototype "RAID levels", or combinations of disks. Each had theoretical advantages and disadvantages. Over the years, different implementations of the RAID concept have appeared. Most differ substantially from the original idealized RAID levels, but the numbered names have remained. This can be confusing, since one implementation of RAID 5, for example, can differ substantially from another. RAID 3 and RAID 4 are often confused and even used interchangeably.

The very definition of RAID has been argued over the years. The use of the term redundant leads many to split hairs over whether RAID 0 is a "real" RAID type. Similarly, the change from inexpensive to independent confuses many as to the intended purpose of RAID. There are even some single-disk implementations of the RAID concept. For the purpose of this article, we will say that any system which employs the basic RAID concepts to recombine physical disk space for purposes of reliability, capacity, or performance is a RAID system.

A hard disk platter is a component of a hard disk drive, it is the circular disk on which the magnetic data is stored. The rigid nature of the platters in a hard drive is what gives them their name (as opposed to the flexible materials which are used to make floppy disks). Hard drives typically have several platters which are mounted on the same spindle. Platters are typically made from aluminium or glass, plastics are rarely used. A thin layer of either iron oxide or another material that possesses similar magnetic properties coats each side of a hard disk platter in order to store the magnetic information. The hard drive heads move over the surface of the platters and read or write the data.

A hard disk drive (HDD, or also hard drive) is a non-volatile data storage device that stores data on a magnetic surface layered onto hard disk platters. A hard disk uses platters (disks). Each platter has a planar magnetic surface on which digital data may be stored. Information is written to the disk by transmitting an electromagnetic flux through a read-write head that is very close to a magnetic material, which in turn changes its polarization due to the flux. The information can be read by a read-write head which senses electrical change as the magnetic fields pass by in close proximity as the platter rotates.

A typical hard disk drive design consists of a central axis or spindle upon which the platters spin at a constant rotational velocity. Moving along and between the platters on a common armature are read-write heads, with one head for each platter surface. The armature moves the heads radially across the platters as they spin, allowing each head access to the entirety of the platter.

A megabyte is a unit of information or computer storage equal to approximately one million bytes. Megabyte is commonly abbreviated as MB (not to be confused with Mb, which is used for megabits), and sometimes as meg.

FireWire (also known as i.Link or IEEE 1394) is a personal computer (and digital audio/video) serial bus interface standard, offering high-speed communications and isochronous real-time data services. FireWire has replaced SCSI in many applications due to lower implementation costs and a simplified and more adaptable cabling system.

Almost all modern digital camcorders have included this connection since 1995. Many computers intended for home or professional audio/video use have built-in FireWire ports including all Macintosh and Sony computers currently produced. FireWire was also an attractive feature on the Apple iPod for several years, permitting new tracks to be uploaded in a few seconds and also for the battery to be recharged concurrently with one cable. However, Apple has phased out FireWire support, in favour of USB 2.0.

Perpendicular Magnetic Recording System
Conventional longitudinal recording stores data on a magnetic disk as microscopic magnet bits aligned in plane. Although advances in magnetic coatings continue to improve data recording densities on HDD, the magnetic bits repulse each other due to in-plane alignment. Squeezing more bits on to a disk will eventually reach a point where crowding degrades recorded bit quality. This places fast-approaching limits on storage capacities. By standing the magnetic bits on end, perpendicular recording reinforces magnetic coupling between neighbouring bits, achieving stable higher recording densities and improved storage capacity.

Toshiba's new HDDs achieve the highest areal density yet reported, 206 megabits per square millimetre*3 (133 gigabits per square inch). The 40GB platter capacity is 33%*4 more than that of Toshiba's conventional HDD.

In computer hardware, Serial ATA (SATA or S-ATA) is a computer bus technology primarily designed for transfer of data to and from a hard disk. It is the successor to the legacy Advanced Technology Attachment standard (ATA, also known as IDE or Integrated Drive Electronics). This older technology was retroactively renamed Parallel ATA (PATA) to distinguish it from Serial ATA.

SCSI stands for "Small Computer System Interface", and is a standard interface and command set for transferring data between devices on both internal and external computer buses. SCSI is usually pronounced "scuzzy".

SCSI is most commonly used for hard disks and tape storage devices, but also connects a wide range of other devices, including scanners, printers, CD-ROM drives, CD recorders, and DVD drives. In fact, the entire SCSI standard promotes device independence, which means that theoretically SCSI can be used with any type of computer hardware.

Since its standardization in 1986, SCSI has been commonly used in the Apple Macintosh and Sun Microsystems computer lines. It has never been popular in the IBM PC world, due to the lower cost and adequate performance of its ATA hard disk standard. The introduction of USB, FireWire, and ATAPI made SCSI a relatively unattractive proposition on PC due to its high cost and rising complexity.

At this time, SCSI is popular on high-performance workstations, servers, and high-end peripherals; and RAID arrays on servers almost always use SCSI hard disks. Desktop computers and notebooks more typically use the ATA/IDE or the newer SATA interfaces for hard disks, and USB or FireWire connections for external devices. 

Universal Serial Bus (USB) provides a serial bus standard for connecting devices, usually to computers such as PCs and the Apple Macintosh, but is also becoming commonplace on video game consoles such as Sony's PlayStation 2, Microsoft's Xbox 360, Nintendo's Revolution, and PDAs, and even devices like televisions and home stereo equipment. USB supports three data rates.

A Low Speed rate of 1.5 Mbit/s (183 KiB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice and joysticks.

A Full Speed rate of 12 Mbit/s (1.4 MiB/s). Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices. All USB Hubs support Full Speed. A Hi-Speed rate of 480 Mbit/s (57 MiB/s).

Though Hi-Speed devices are commonly referred to as "USB 2.0", not all USB 2.0 devices are Hi-Speed. A USB device should specify the speed it will use by correct labelling on the box it came in or sometimes on the device itself. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or High-Speed after passing a compliancy test and paying a licensing fee. All devices are tested according to the latest spec, so recently-compliant Low Speed devices are also 2.0.

Hi-Speed devices should fall back to the slower data rate of Full Speed when plugged into a Full Speed hub. Hi-Speed hubs have a special function called the Transaction Translator that segregates Full Speed and Low Speed bus traffic from Hi-Speed traffic. The Transaction Translator in a Hi-Speed hub (or possibly each port depending on the electrical design) will function as a completely separate Full Speed bus to Full Speed and Low Speed devices attached to it. This segregation is for bandwidth only; bus rules about power and hub depth still apply.