A computer Hard Drive, or Fixed Disk, is where all your data is stored when a computer is off. Because the data is stored magnetically, it doesn't need any electrical power to retain or remember all this data.

PHYSICAL CONSTRUCTION.

The Hard Drive consists of usually one metal disk coated on both sides with a special magnetic material, usually an iron oxide compound, that has tiny particles that can be magnetically orientated to represent bits - which are the basic unit of computer memory, and are equivalent to a switch, being either on or off.

This disk then spins at a very high rate of 8000 revolutions per minute or more, and then the two read/write heads that are used to actually retrieve the data from this disk, skim just a few hundredths of a millimetre from the surface of this spinning disk. The read/write heads are suspended on the end of metal arms that are able to move from the outer edge to the inner edge of this disk, so that they can seek out the data that is required. Very powerful rare earth magnets help control the rapid movement of these heads.

HOW DATA IS STORED ON THE HARD DRIVE.

The data on a HDD is arranged in concentric tracks, almost microscopic in width, and the disk is further divided into sectors, like slices of a pizza, and on hard drives that have more than one disk, or platter, cylinders are also defined. A cylinder is all the tracks, on multiple platters, that are the same circumference.

When a disk is still raw, it needs to be formatted, which is the process whereby the disk is divided magnetically into these tracks, sectors and cylinders so that the data can be written to the right places. A boot record is also written onto the disk during formatting, which is a track containing information about how the disk is formatted and constructed.

At a spin rate of 8000 RPM, the outer edge of the disk is moving under the heads at a speed of over 100km/h, or over 60 mph. If the heads were to touch the surface at this speed, some serious damage would occur. This sometimes happens when a hard drive gets a hard jolt, and this is known as a head crash. Data is transferred by means of the heads being given an instruction to read or write to the magnetic disk surface at the precise moment the head flies over that part of the disk. Thus the head has about one 2,000th of a second to do it's job of writing a block or segment of data.

In the old days of computers, the computer itself had to control where the data was put on a HDD. In modern computers, hard drives come with built in IDE, or Integrated Drive Electronics, which takes care of all the nitty gritty of random data storage, leaving the computer CPU to get on with other important work. This system speeds up the computer operation overall, by sharing the workload.

Hard drives can store large amounts of data. The most common drive size in computers at the moment is in the region of 120 Gigabytes, which is enough space to store the text of about 150,000 best seller novels. Laptops are now coming out with 1 terabyte hard drives, which is 1,000 Gigabytes of storage. Most desktop computers can easily accommodate 4 hard drives, so loading the text of all the books in your local city library onto your personal computer hard drives is quite possible.

Although a Hard drive doesn't look like much, it is a remarkable piece of precision engineering married to lightning fast electronics which has been constantly improved since the first hard drives of 5 megabytes came out in the 1980's

Hard drives have certainly come a long way.

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