Buy External, EXT, Internal Computer Hard Drive from Seagate, Western Digital, Iomega, Maxtor for your laptop and Desktop only at ebazaar.biz!

How to search: To search for Terabyte Hard Drive type in "terabyte hard drive" in search box below

Enter your search terms Submit search form

A hard disk drive ( HDD )

commonly referred to as a hard drive or hard disk , is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media.

HDDs were originally developed for use with computers . In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders , digital audio players , personal digital assistants , digital cameras , and video game consoles . In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia . The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

The magnetic surface of each platter is divided into many small sub- micrometre -sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early HDDs used the same inductor that was used to read the data as an electromagnet to create this field. Later versions of inductive heads included, metal in Gap (MIG) heads and thin film heads. In today's heads the read and write elements are separate but are in close proximity on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.

Hard disk drives are sealed to prevent dust and other sources of contamination, from interfering with the operation of the hard disks heads. The hard drives are not air tight, but rather utilize an extremely fine air filter, to allow for air inside the hard drive enclosure. The spinning of the platters causes the air to circulate forcing any particulates to become trapped on the filter. The spinning of the platters, also allows the hard disk heads to float above the surface of the disk surface using the same air currents. See Bernoulli's principle .

Using rigid platters and sealing the unit allows much tighter tolerances than in a floppy disk drive . Consequently, hard disk drives can store much more data than floppy disk drives and access and transmit it faster. In 2007, a typical enterprise, i.e. workstation HDD might store between 160 GB and 1 TB of data (as of local US market by July 2007), rotate at 7,200 or 10,000 revolutions per minute (RPM), and have a sequential media transfer rate of over 80 MB/s. The fastest enterprise HDDs spin at 15,000 RPM, and can achieve sequential media transfer speeds up to and beyond 110 MB/s. Mobile, i.e., Laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. In the 1990s, most spun at 4,200 RPM. In 2007, a typical mobile HDD spins at 5,400 RPM, with 7,200 RPM models available for a slight price premium.

Hard disk drive characteristics

Capacity of a hard disk drive is usually quoted in gigabytes . Older HDDs quoted their smaller capacities in megabytes .

The data transfer rate at the inner zone ranges from 44.2 MB /s to 74.5 MB/s, while the transfer rate at the outer zone ranges from 74.0 MB/s to 111.4 MB/s. An HDD's random access time ranges from 5 ms to 15 ms.

The physical size of a hard disk drive is quoted in inches . The majority of HDDs used in desktops today are 3.5" wide, while those used in laptops are 2.5" wide. As of early 2007, manufacturers have started selling SATA and SAS 2.5 inch drives for use in servers and desktops.

An increasingly common form factor is the 1.8" ATA-7 LIF form factor used inside digital audio players and subnotebooks , which provide up to 100GB storage capacity at low power consumption and are highly shock-resistant. A previous 1.8" HDD standard exists, for 2–5GB sized disks that fit directly into a PC card expansion slot. From these, the smaller 1" form factor was evolved, which is designed to fit the dimensions of CF Type II, which is also usually used as storage for portable devices including digital cameras . 1" was a de facto form factor led by IBM 's Microdrive , but is now generically called 1" due to other manufacturers producing similar products. There is also a 0.85 inch form factor produced by Toshiba for use in mobile phones and similar applications, including SD / MMC slot compatible HDDs optimized for video storage on 4G handsets.

The size designations are more nomenclature than descriptive. The names refer to the width of the disk inserted into the drive rather than the actual width of the entire drive. A 5.25" drive has an actual width of 5.75", a 3.5" drive 4", a 2.5" drive 2.75". A 1.8" drive can have different widths, depending on its form factor. A PCMCIA drive has a width of 54 mm, while an ATA-7 LIF form factor drive has a width of 2.12".

A hard disk is defined to be at "full height" if its height is 3.25". It is "half height" at a height of 1.625". A "slim height" or "low profile" HDD has a height of 1". "Ultra low profile" drives can have heights of 0.75", 0.67", 0.49" or 0.37".

Hard disk drives are accessed over one of a number of bus types, including ATA (IDE, EIDE), Serial ATA (SATA), SCSI , SAS , and Fibre Channel . Bridge circuitry is sometimes used to connect hard disk drives to busses that they cannot communicate with natively, such as IEEE 1394 and USB .

Back in the days of the ST-506 interface, the data encoding scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate by fifty percent, to 7.5 megabits per second; it also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.)

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

SCSI originally had just one speed, 5 MHz (for a maximum data rate of 5 megabytes per second), but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early IBM PC compatibles and early Apple Macintoshes .

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run in a master/slave setup (two disks on the same cable). This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself.

Manufacturers

The technological resources and know-how required for modern drive development and production mean that as of 2007, over 98% of the world's HDDs are manufactured by just a handful of large firms: Seagate , Western Digital , Samsung , and Hitachi (which owns the former disk manufacturing division of IBM ). Fujitsu continues to make mobile- and server-class disks but exited the desktop-class market in 2001. Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.

Terabyte

terabyte (derived from the prefix tera - and commonly abbreviated TB ) is a measurement term for data storage capacity . The value of a terabyte is based upon a decimal radix (base 10) and is defined as one trillion ( short scale ) bytes , or 1000 gigabytes .

The number of bytes in a terabyte is sometimes stated to be approximately 1099 x 10 9 . This difference arises from a conflict between the long standing tradition of using binary prefixes and base 2 in the computer world, and the more popularly and intuitive decimal ( SI ) standard adopted widely in the industry. Standards organizations such as IEC , IEEE and ISO recommend to use the alternative term tebibyte (TiB) to signify the traditional measure of 1024 4 bytes, or 1024 gibibytes , leading to the following definitions:

• According to the SI standard and current usage, a terabyte (TB) contains 1,000,000,000,000 bytes = 1000 4 or 10 12 bytes.

• According to traditional and binary arithmetic, a terabyte contained 1,099,511,627,776 bytes = 1024 4 or 2 40 bytes. This amount is now known instead as a tebibyte, to avoid confusion.

The capacities of computer storage devices are traditionally advertised using their SI standard values.