Hard Disk Drives

Replacing the original hard disk or adding a hard disk is one of the most common upgrades. It's easy to do and a very cost effective way to extend the life of a PC. This chapter explains what you need to know to choose, install, and configure hard disks and interface.

How Hard Disks Work

All hard disks are constructed similarly. A central spindle supports one of more platters, which are thin, flat, circular objects made of metal or glass, substances chosen because they are rigid and do not expand and contract much as the temperature changes. Each platter has two surfaces, and each surface is coated with a magnetic medium. Most drives have multiple platters mounted concentrically on the spindle, like layers of a cake. The central spindle rotates at several thousand revolutions per minute, rotating the platters in tandem with it.

A small gap separates each platter from its neighbors, which allows a read-write bead mounted on an actuator arm to fit between the platters. Eachsurface has its own read-write head, and those heads "float" on the cushion of air caused by the Bemoulli Effect that results from the rapid rotation of the platter. When a disk is rotating, the heads fly above the surfaces at a distance of only millionths of an inch. The bead actuator assembly resembles a comb with its teeth inserted between the platters, and moves all of the heads in tandem radially toward or away from the center of rotation.

The small separation between the heads and surfaces means that a tiny dust particle could cause a catastrophic  head crash, so these components are sealed within a bead/disk assembly, or HDA.The sealed HDA contains air filters that allow air pressure to equalize between the HDA and the surrounding environment. Opening an HDA other than in a factory clean-room is a certain way to destroy a disk drive.

Each surface is divided into concentric tracks that can be read from of written to by that surface's head.Each surface on a modem disk drive contains thousands of tracks. Each track is divided into many sectors, which store 512 bytes of data. Old drives used the same number of sectors on every track, typically 17 or26. Modern drives take advantage of the fact that tracks near the outer edge of the platter are longer than those near the center by storing more sectors on the outer tracks.

All tracks that are immediately above and below each other form a cylinder. If a drive has eight surfaces, each with 2,048 tracks, that drive contains 2,048 cylinders, with eight tracks per cylinder. The concept of cylinders is important because it determines how data is written to and read from the drive. When a drive writes a file that is larger than one track, it fills the current track and then writes the reminder of the file sequentially to the next available track within that cylinder Only if the capacity of the current cylinder is exceeded does the drive move the heads to the next available cylinder. The drive writes data in this fashion because selecting a different read-write head is an electronic operation, which occurs quickly, while moving the heads to a different track is a mechnical operation that requires significantly more time.