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Every PC has at least one -- many have more than one. Camcorders, video recorders MP3 players and even digital cameras now have them. The hard disk is one of the most important parts of a PC. It provides permanent, large-scale storage in the PC, retaining information even when the PC is powered off (unlike the memory). It's where the operating system, programs and documents on your PC are stored permanently.

If you're buying or building a new PC, or have simply run out of space on your existing PC and need to boost your storage capacity, knowing what to look for in a hard disk is vital.

How do they work?

Originally conceived in the 1950s, a hard disk comprises of one or more circular platters of magnetic material, with billions of microscopic magnetic domains, each representing one bit of data. These zones are embedded into concentric circles, called tracks. A read/write head floats above the tracks, and the magnetic flux patterns of the zones affects an electric current flowing through the head, which in turn registers as a "0" or a "1" to the drive's electronics. The head can also alter the magnetic patterns, to write data onto the disk.

When in operation, an electric motor spins the hard disk constantly, rather like an LP or CD, in order to bring the data on the tracks around to the head. The head moves across the surface of the disk, to position it above the track on which the desired data is stored. The head will then wait for the rotation of the disk to bring the data around.

A "hard disk", as we understand it today, will usually comprise multiple platters (typically three in a standard consumer hard disk), packaged in a standard-size casing. Desktop hard disks for instance, are usually packaged to go into 3.5in slots in your computer case.

Capacity

For most users, the most important feature of a hard disk is how much data it can store. Recently released hard disks can store up to 400GB on a single 3.5in disk. Most people don't need that much space, however, and more mainstream disks tend to float around the 120GB-200GB mark. The going rate for capacity tends to be around $1 per gigabyte, with significant variation for premium and ultra-high capacity products.

The planned use of a PC's hard disk will determine how much space is required. In consumer PCs, much of the space on a PC's hard disk will be consumed by media files. If you plan to store a lot of media - especially video, but also audio - you need a big hard disk (especially if you plan to leave the media on the hard disk permanently, rather than archive it to DVD or tape). The following list should give you a rough estimate of the amount of space each application will take on a hard disk.

• The operating system: 1-2GB.
• An office suite: 1GB.
• A fully installed computer game: 200MB-2GB or more.
• A CD directly copied to hard disk: 400-700MB.
• A single MP3 music file: 5MB.
• A full album of MP3 files: 60MB.
• An un-recompressed DVD (that is, directly copied to hard disk): 4-8GB.
• A DVD movie "ripped" (that is, compressed) to DivX, such as a downloaded movie: 700MB - 1.4GB.
• However, it is worth mentioning - there are legal consequences surrounding the downloading of movies.
• Although rampant, it is illegal to copy commercial movies.

One constant source of confusion for buyers of hard disks is the disparity between the advertised size of the hard disk and the capacity that is reported by the operating system. More than one buyer has felt ripped-off when they installed their shiny new 80GB hard disk, for instance, only to find that the operating system tells them that the disk only has a 74.5GB capacity.

This is not actually an error, but an holdover of the different ways in which the hard drive vendors and the software systems measure capacity. When calculating their disk size, hard drive vendors use the decimal definition of gigabyte. That is, one gigabyte equals 1000 megabytes; one megabyte equals 1000 kilobytes and so on.

The operating system, however, will use the binary definition of a gigabyte when calculating available space. In binary terms, one gigabyte equals 2^10, or 1024 megabytes. One megabyte is 1024 kilobytes.

So, using a decimal definition (as the hard drive vendors do, for obvious reasons), a hard disk with 80,000,000,000 bytes of storage is 80GB. But in binary terms, that's a little less than 75GB.