Hard Disk Drive

A hard disk drive (HDD) is a non-volatile, random access device for digital data. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the platters.

Introduced by IBM in 1956, hard disk drives have fallen in cost and physical size over the years while dramatically increasing in capacity. Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s. They have maintained this position because advances in their areal recording density have kept pace with the requirements for secondary storage. Today's HDDs operate on high-speed serial interfaces; i.e., serial ATA (SATA) or serial attached SCSI (SAS).

Capacity measurements

The capacity of hard disk drives is given by manufacturers in megabytes(1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) orterabytes (1 TB = 1,000,000,000,000 bytes). This numbering convention, where prefixes like kilo- and mega- denote powers of 1000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM,ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1024.

When the unit prefixes like kilo- denote powers of 1024 in the measure of memory capacities, the 1024ⁿ progression (for n = 1, 2, …) is as follows:

• kilo = 2¹⁰ = 1024¹ = 1024,
• mega = 2²⁰ = 1024² = 1,048,576,
• giga = 2³⁰ = 1024³ = 1,073,741,824,
• tera = 2⁴⁰ = 1024⁴ = 1,099,511,627,776

Data transfer rate

As of 2010, a typical 7200 rpm desktop hard drive has a sustained "disk-to-buffer" data transfer rate up to 1030 Mbits/sec.^[62] This rate depends on the track location, so it will be higher for data on the outer tracks (where there are more data sectors) and lower toward the inner tracks (where there are fewer data sectors); and is generally somewhat higher for 10,000 rpm drives. A current widely used standard for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file. Transfer rate can be influenced by file system fragmentation and the layout of the files.^[57]

HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates.^{[citation needed]} Areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, the latter will increase the data transfer rate (for a given RPM). Since data transfer rate performance only tracks one of the two components of areal density, its performance improves at lower rate