5. Hard Disk

The hard disk is the main storage device in your computer. It is a bit like a filing cabinet: all of your data files and applications software are stored on it.

The hard disk contains a number of metal platters which have been coated with a special magnetic material.

The data is stored in this magnetic material. Thus, the hard disk is known as a magnetic storage device.

In order to access the data, the platters spin many thousands of times a second and a magnetic read and write head floats just above the surface of the platter.

When you hear the term 'hard disk crash', this refers to the read/write head crashing down onto the surface of the hard disk. There is a risk every time this happens that the data stored in the section just where the head crashes might be damaged. That is why it is a bad idea just to switch the computer off at the wall without shutting it down properly.

Hard disks are measured in Gigabytes and Terabytes. Typical hard disks range from around 120 Gb - 3 Terabytes.

Hard disks also come as stand-alone external drives, as shown opposite.

Hard disks are now used in non-computer equipment as well. For example the PVR (Personal Video Recorder) such as the Sky satellite box can store many hours of programs because they have a hard disk inside.

Advantages

• Necessary to support the way your computer works
• Large storage capacity
• Stores and retrieves data much faster than a floppy disk or CD/DVD
• Stored items are not lost when you switch off the computer
• Cheap on a cost per megabyte compared to other storage devices

Disadvantages

• Far slower to access data than ROM or RAM chips
• Hard disks can crash which stop the computer from working
• Regular crashes can damage the surface of the disk, leading to loss of data in that sector
• The disk is fixed inside the computer and cannot easily be transferred to another computer.