A hard disk or drive is the part of your computer responsible for long-term storage of information. Unlike volatile memory (referred to as RAM) which loses its stored information once the power supply is shut off, a hard disk stores information permanently, allowing you to save programs, files, and other data. Hard disks also have much greater storage capacities than RAM; in fact, current hard disks contain over 100 GB of storage space!

Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called "fixed disks" or "Winchesters" (a code name used for a popular IBM product). They later became known as "hard disks" to distinguish them from "floppy disks." Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.

At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use essentially the same magnetic recording techniques. Hard disks and cassette tapes also share the major benefits of magnetic storage - the magnetic medium can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years.

The following are the major differences between cassette tapes and hard disks:

• The magnetic recording material on a cassette tape is coated onto a thin plastic strip. In a hard disk, the magnetic recording material is layered onto a high-precision aluminum or glass disk. The hard disk platter is then polished to mirror smoothness.
  
  
• With a tape, you have to fast-forward or reverse through the tape to get to any particular point on the tape. This can take several minutes with a long tape. On a hard disk you can move to any point on the surface of the disk almost instantly.
  
  
• In a cassette tape deck, the read/write head touches the tape directly. In a hard disk the read/write head "skims" over the disk surface, never actually touching it.
  
  
• The tape in a cassette tape deck moves over the head at about 2 inches (about 5.08 cm) per second. A hard disk platter can spin underneath its head at speeds up to 3,000 inches per second (about 170 mph or 272 kph)!
  
  
• The information on a hard disk is stored in extremely small magnetic domains as compared to a cassette tape. The size of these domains is made possible by the precision of the platter and the speed of the media.

Because of these differences, a modern hard disk is able to store an amazing amount of information in a small space. A hard disk can also access any of its information in a fraction of a second.

A typical desktop machine will have a hard disk with a capacity of between 20 and 40 gigabytes. Data is stored onto the disk in the form of files. A file is simply a named collection of bytes. The bytes might be the ASCII codes for the characters of a text file, or they could be the instructions of a software application for the computer to execute, or they could be the records of a data base, or they could be the pixel colors for a GIF image. No matter what it contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time.

There are various ways to measure the performance of a hard disk:

• Data rate - The number of bytes per second that the drive can deliver to the CPU. Typically around 33 to 150 megabytes per second (MB/s).
  
  
• Disk RPM - The maximum revolutions per minute the disk platter can move at. Ranges from 5400 to 10,000 rpm.
  
  
• Seek time - The amount of time it takes between the time that the CPU requests a file and the first byte of the file starts being sent to the CPU. Times between 10 and 20 milliseconds are common.
  
  
• Drive capacity - The number of bytes it can hold. Typically upto 200 GB.
  
  
• Cache buffer size - The size of the hard disk's internal cache that buffers data being transferred from the disk to the motherboard. Data is transferred from the motherboard to the cache and vice-versa over the ATA/IDE cable. For reads, data is read from the platter to the cache before being sent out and vice-versa for writes. Typically 2 MB.