Nearly every desktop computer and server in use today contains one or more for of internal storage. Every server normally has several of them. You can even find VCR-type devices and camcorders that use hard disks instead of tape.

These billions of hard disks do serve one main purpose and that is store digital information in a relatively permanent form. They give computers the ability to retain data when the power goes out. 

The term Hard disk was probably derived from the fact that, they are physical and robust discs. They 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 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 retain the magnetic flux patterns stored onto the medium for many years.
The big differences between cassette tapes and hard disks may be:

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-type smoothness.

With a tape, you have to fast-forward or reverse 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 "flies” over the disk, 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 272 KPH)! 

The information on a hard disk is stored in extremely small magnetic domains compared to a cassette tape’s. The size of these domains is made possible by the precision of the platter and the speed of the medium.

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 40 and 250 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 two ways to measure the performance of a hard disk:  Data rate - The data rate is the number of bytes per second that the drive can deliver to the CPU.

Rates between 5 and 40 megabytes per second are common.

Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.

The other important parameter is the capacity of the drive, which is the number of bytes it can hold.

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