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.

hard disk

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.

There may even be several hard disks fitted inside the computer.

Properties of a Hard Disk
Property Notes
Type of storage Magnetic
Data access Direct access (i.e. not serial access like a tape)
Cost of storage Hard disks hold a vast amount of data and are relatively cheap, so on a per-byte basis they are the amongst the cheapest form of storage, although magnetic tape is cheaper.
Capacity Can be a Terabyte or more
Speed An internal hard disk is faster than external storage devices. It is however, slower than internal solid state memory (SSD) which are now appearing in top end computers

An internal hard disk is not intended to be portable - it is screwed to the computer chassis as a permanent fixture unless a replacement is needed.

An external hard disk used for backing up data is portable as it comes in various pocket-sized or larger cases, but it is larger than a DVD or USB memory stick.


Very durable, it can last for years if treated carefully.

Can be damaged by being dropped, experience extreme heat or strong magnetic fields.

Data can be written to it and deleted from it an infinite amount of times

Reliability As long as it is not damaged, a hard disk is very reliable., The system detects any failed area on the disk surface and prevents data being written in that spot (these are called 'bad sectors').

Challenge see if you can find out one extra fact on this topic that we haven't already told you

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