What Does the Hard Drive Do?

Hard drives are at the center of computers and countless other products. Nowadays, camcorders, music players, camcorders and many other types of personal electronic devices store information on hard drives too. Although manufacturing hard drives is a precise and exacting process, the basic principles of hard drive function are surprisingly simple.

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A hard drive is a device used to store data in a semipermanent state on your computer. Computer memory is a series of small packets of data called bits. Each bit is a 1 or a 0, and each bit is stored as a small magnetic charge on the hard drive. A read/write head moves over the hard drive to retrieve or rewrite the bits as needed.

Hard Drive Structure

The hard drive contains several smooth, metal-coated disks called platters. Each platter is divided into a number of narrow, concentric circles called tracks, which run from the the inside of the platter all the way to the outer edge. Each track is further subdivided into small sections called blocks. To retrieve a piece of information, the head moves to the exact track and sector on the correct platter.

Hard Drive Mechanics

Hard disk platters spin at a constant speed of several thousand revolutions per minute (the exact speed depends on the drive). The read/write head is also very quick, and some can move from the outer to the inner edge of the disk 50 times per second. The head is located on an arm that is actuated by a device called a voice coil. The voice coil has both an electromagnet and a permanent magnet. When the electromagnet turns on, it pushes against the permanent magnet, moving the head arm across the disk. The more power, the farther the head moves. When the electromagnet is turned off, a spring pulls it back across the drive.

Reading and Writing

The read/write head is actually two devices in one--a read head and a write head. The read head contains a small coil or some other device that is sensitive to magnetic flux. When the head moves past a charged part of the hard drive, the magnetic field creates a small charge in the read head, which the head registers as a bit. The write head has a small electromagnet in it. As it moves past a part of the platter where it wants to write data, it produces a small magnetic field. The sensitive platter picks up and stores this magnetic field as a bit.


Many people assume that a hard drive will last forever, but actually hard disks are extremely fragile pieces of hardware prone to sudden failure. The head sits very close to the high-speed platter. A sharp jolt can make it bump into the platter, scratching and ruining the hard drive. Any dust or debris leaking into the hard drive can also damage the platter. The data on the drive can also be ruined by a magnetic field, which will overwrite the store bits.

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