The Inner World of the Hard Drive

When people think of personal computers, they almost always think of the CPU, or memory, or even graphics chips. They often forget that inside a PC is a purely mechanical device that spins at up to 15,000 revolutions per minute. In a more standard desktop hard drive, which spins at a tamer 7,200 RPM, a point close to the outer track is moving at roughly 48 miles per hour. In a typical working day, that means the outer edge of your hard drive has traveled 384 miles.

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But thats not the most amazing part. Each hard drive consists of one or more platters, called the substrate, typically made from very thin glass. Were not talking window glass here, but a precisely formulated glass or glass composite, which is highly polished and coated with a thin layer of magnetic material. The drive can record data on both sides of a platter by orienting magnetic domains within the material. One direction represents a digital 1, the other a 0. In modern high-capacity drives, Magnetic domain orientation is often vertical (pointing up or down).

The key to all this is the heads. While the material making up the platters and magnetic media is certainly important, the read and write functions are handled by the heads, which fly barely one micron above the platter surface. To put this in context, a human hair is roughly 200 microns in diameter. Keep in mind that the platter is moving beneath the head at 7,200 RPM—thats like flying a jet about a foot above the ground. Since drives today can have up to five platters, each with two recording surfaces, you can have as many as ten heads, each hovering over its own surface. Multiple heads are part of a single mechanism, though; they cannot move independently. This makes them easier to control.

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