Anatomy of a Hard Disk

By Russell Kay

October 15, 2001 12:00 PM ET

Computerworld - Almost all computers today store their digital data as magnetic areas on a device called a hard disk, hard drive or fixed disk.

Basically, all hard drives work the same way: Information is encoded and "written" onto a round, spinning aluminum or glass platter that's been coated with magnetic material. The writing is done by a magnetic head, mounted at the end of an arm that pivots in such a way that the head can be positioned over any part of the platter. The same head also reads the stored data. Special software or firmware on the disk drive and the computer keep track of where any piece of information is stored. Older disk drives devoted one entire side of one platter, along with its head, as a servomechanism to calibrate and regulate platter and arm motion, but current technology doesn't require nearly so much space.

Remember when music came on vinyl records? A disk drive operates a lot like the phonograph. Each has a motor that spins a platter containing information that is written or retrieved by a special device mounted on the end of an arm that pivots across the disk.

There are considerable differences, of course. The LP record was plastic and 12 in. in diameter, and it spun at 33-1/3 rpm. The computer hard drive, once 14 in. or more across, is now no bigger than 3.5 or 5.5 in. in diameter, with those in laptops and handheld devices at 2.5, 1.8 or even 1 in. Hard disks spin at speeds ranging from about 4,000 to 15,000 rpm, and those speeds are likely to increase in the future. And where the phonograph needle physically touched the record groove, the drive heads don't touch the spinning media at all, though they get very close while flying on a cushion of air.

Today's disks can store immense amounts of data: About the smallest 3.5-in. hard drive being made today will store 10GB, and capacities for individual drives have reached 100GB. Drive makers have two ways of increasing the capacity of a disk drive. The simplest method is to add additional platters along with a separate head for each side of each platter, and this has been done up to about 16 platters. The second, more basic, way is to increase the amount of data that can be stored on a single area of the magnetic material. This has been the subject of considerable research. Today, IBM has drives that store 25.7GB per square inch, and the company has demonstrated technologies that can quadruple that, to 100GB of data in a single square inch.

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