The Hard Disk

By Walton Dell
December 28, 1998


I recommend you first read my tutorial on Windows Explorer.

What is a hard disk?

The hard disk in your computer allows you to store dozens of programs and thousands of documents all inside your computer.  A modern hard disk allows many billions of characters of text to be stored within a device no larger than a sandwich. 


How is information stored on a hard disk?

Computers use the binary number system (just ones and zeros) to represent data.  However, a computer rarely works with just one bit at a time.  Instead, the computer groups 8 bits together to form a byte.   Bytes can represent any type of data, including images and sound, but the simplest data type is ASCII text.  ASCII text is simply one byte per character of text (including spaces and symbols).  If your hard disk can store 7 gigabytes, then it could hold 7 billion characters of text (if it held nothing else).  One very important type of data is program data.  In files of this type, the bytes represent computer instructions.

At a pre-defined location on the hard disk, there is an organized set of data called the File Allocation Table (FAT).  The FAT keeps track of the starting location on the disk for each file.


Why was the hard disk invented?

In the early days of computing, you would have to put a disk (or tape) into a computer to do anything at all.  If you wanted to run a different program, you would have to take out the current disk (or tape), then put in the one you wanted to use.   A computer like this would be useless without a disk in the drive.  The hard disk was invented so that large amounts of data (including programs) could be stored inside a computer.


What is a hard disk, mechanically?

A hard disk is actually made up of multiple rigid disks within an airtight metal case.  These disks spin at a very high speed (between 3600 and 10000 revolutions per minute).  Hovering over each disk is a "head" which reads or writes data with electromagnetic pulses.

If you are a client of mine, I can show you, during our next session, an old hard disk I have taken apart.  Please ask.

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