Hard Disk

Disks are the most commonly used type of storage. There is a wide variety of different disk types, including many sizes and formats of floppy disks, hard disks, optical disks, CD-ROMs (Compact Disc Read-Only Memory), and removable hard disks (such as Syquest).

In general, all sorts of disk storage share certain common elements. On all disks, physical differences in the surface of the disk are used to represent data. On  floppy and hard disks, magnetism is used to encode data. On CD-ROM and optical disks, variations in how the disk surface reflects light are used to encode data. Here we discass only the hard disks.

Disks arrange information into concentric rings called tracks. Tracks are divided into pie-like slices called sectors. Some disks can be written to only on one side; others can be written to on both sides. A read/write head can be positioned over any track, and data is read (or written) as the sectors pass by.

Hard drives, or fixed disks, are a type of storage device that provide fast access to large amounts of storage in a small, reasonably reliable physical package. Without them, most modern computing applications would be impossible. Hard disks are often composed of multiple disks. A cylinder consists of a track on the top side of the top-most disk, and all of the tracks beneath it, as shown in Figure. A cylinder represents all of the data that the read/write heads can access when they are in a certain position. (There is a separate read/write head for each side of each disk, but they all move together.)

Hard drives have been designed to meet users’ needs for speed and capacity. With the maturation of the technology, designers now add reliability to, and reduce the cost of, the design process. This constant redesign process has produced better drives, in many different types. However, even with differences,

almost all hard drives operate the same way: data is stored as locations of magnetic flux, or change, on a disk of specially coated aluminum or glass. Hard disks can have one or more of these platters or disks. The information is read or written with a head, or small magnet, that floats on a cushion of air over the platter. The platter spins at a high rate, generally 5,400 or 7,200 revolutions per minute (rpm). The heads are moved across the platter by one of two technologies: older designs used a motor, called a stepper motor, that moved only in pre-defined increments, or steps. Newer designs use a voice-coil, similar to an audio speaker, to move the heads more precisely over the platter.

Writing Data to the Hard Disk

Hard disks spin at very fast speeds, and the read/write heads hover over the platters, very close to the surface so that they can read or write data. The platters are made of a rigid material, such as aluminum, that is coated with a magnetic material. To write data, the computer positions the head in a particular track. When the appropriate sector passes by, pulses of electricity are sent through a coil of wire in the head. This creates an electromagnetic field, which aligns magnetic particles on the disk surface. By alternating the flow of the current to the head, ones and zeroes can be encoded magnetically.

The data is encoded, or written, in circular tracks as the head floats over the rotating platter. Each platter has its own read/write head. The newer voice-coil designs allow cylinders to be written closer together so more data can be fit onto the same-sized platter

Reading Data from the Hard Disk

To read data, the computer positions the head over the appropriate track. When the sector passes by, the magnetic particles on the disk create an electrical current in the head through a phenomenon known as inductance. In the head, the alternating patterns of magnetism on the disk translate into alternating flows of electrical current, which can be translated into ones and zeroes.

Avoiding Head Crash

You should never transport or jar a hard disk that is spinning, because you can easily cause a head crash. If the read/write heads bang against the surface of the disk, you might damage that part of the disk, and possibly the read/write heads as well. Most hard-disk drives automatically park over an unused section of the disk when the computer is switched off. Hard-disk drives are tightly sealed to prevent dust and other particles from entering the drive. A single dust particle is likely to be larger than the gap between the

head and the disk platter. With the platter spinning, dust acts like sandpaper on the surface of the disk. For this reason, you should never break the seal on a hard-disk drive.