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A round plate on which data can be encoded. There are two basic types of disks: magnetic disks and optical disks.

On magnetic disks, data is encoded as microscopic magnetized needles on the disk's surface. You can record and erase data on a magnetic disk any number of times, just as you can with a cassette tape. Magnetic disks come in a number of different forms:

  • floppy disk : A typical 5¼-inch floppy disk can hold 360K or 1.2MB (megabytes). 3½-inch floppies normally store 720K, 1.2MB or 1.44MB of data.
  • hard disk : Hard disks can store anywhere from 20MB to more than 200GB. Hard disks are also from 10 to 100 times faster than floppy disks.
  • removable cartridge : Removable cartridges are hard disks encased in a metal or plastic cartridge, so you can remove them just like a floppy disk. Removable cartridges are very fast, though usually not as fast as fixed hard disks.
  • Optical disks record data by burning microscopic holes in the surface of the disk with a laser. To read the disk, another laser beam shines on the disk and detects the holes by changes in the reflection pattern.

    Optical disks come in three basic forms:

  • CD-ROM : Most optical disks are read-only. When you purchase them, they are already filled with data. You can read the data from a CD-ROM, but you cannot modify, delete, or write new data.
  • WORM : Stands for write-once, read-many. WORM disks can be written on once and then read any number of times; however, you need a special WORM disk drive to write data onto a WORM disk.
  • erasable optical (EO ): EO disks can be read to, written to, and erased just like magnetic disks.
  • The machine that spins a disk is called a disk drive. Within each disk drive is one or more heads (often called read/write heads) that actually read and write data.

    Accessing data from a disk is not as fast as accessing data from main memory, but disks are much cheaper. And unlike RAM, disks hold on to data even when the computer is turned off. Consequently, disks are the storage medium of choice for most types of data. Another storage medium is magnetic tape. But tapes are used only for backup and archiving because they are sequential-access devices (to access data in the middle of a tape, the tape drive must pass through all the preceding data).

    A new disk, called a blank disk, has no data on it. Before you can store data on a blank disk, however, you must format it

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