A computer relies on its hard drive to store nearly everything that it needs to operate, including all of its programs and data. Hard drive capacity has steadily increased while the cost and physical size of the drives have decreased at the same time. Constant research into this field of technology may guarantee that what is standard today will probably be replaced by something better tomorrow. At the same time, the general characteristics of the hard drive will remain the same.
The capacity of a hard drive is measured in bytes. Modern drive capacities are in the gigabyte (billions of bytes) and terabyte (trillions of bytes) range and likely to go higher. The capacity is a factor of the number of platters, or disks, that are installed in the drive and the density of the magnetic storage capability of those platters.
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The hard drive is an electro-mechanical device. The data that is stored on the magnetic platters is read by a head that floats just above the surface as the disk rotates beneath it. The read-write head must move to different parts of the platter as it spins to read all of the parts of a file. The combination of the speed of the head movement and how quickly the platter can rotate under the head form the basis for the access speed.
Early hard drives were huge, housed in separate machines and connected to the CPU via heavy cables. Modern hard drives are limited to three physical formats: 3.5-inch, 2.5-inch and 1.8-inch. The smaller physical size limits the number of platters and the diameter of those platters. A 1.8-inch drive, for example, has a maximum capacity of 320 gigabytes.
The electronic connection between the hard drive and processor has undergone a number of changes over time. Each interface change has improved the data transfer speed and ease with which the hard drive is handled by the motherboard in the computer. The current standard interface is SATA, the Serial Advanced Technology Attachment.