3 STORAGE DEVICES AND MEDIA
3.1.2 Hard disk drives (HDD)
Hard disk drives (HDD) are still one of the most common methods used to store data on a computer. Data is stored in a digital format on the magnetic surfaces of the hard disks (or platters, as they are usually called). A hard disk drive will very often have more than one platter depending on its capacity. A number of read/write heads can access all of the platter surfaces in the disk drive (normally each platter will have two surfaces where the data is stored). These read/write heads can move very quickly – typically they can move from the centre of the disk to the edge of the disk (and back again) 50 times a second. Data on an HDD can be read using direct access – this means, unlike magnetic tape, earlier data does not have to be read first before the required data is found. We will now look in more detail at how HDD works:
Actuators are used to move the read/write heads (voice coils are used as the actuators; these are similar to the electromagnets used in speakers – hence their name).
A read/write arm swings the read/write head back and forth across the platter; the platter is rotating at up to 10 000 rpm (revolutions per minute).
Each read/write head contains a tiny magnet which allows the data on the platter to be read.
Platters are made from glass, ceramic or aluminium which are coated in iron oxide.
There are two read/write heads per platter (one for the top surface and one for the bottom surface).
Data is stored in concentric, circular tracks; each track is broken up into sectors (see Figure 3.4).
A map of the sectors is stored on the HDD and is known as a file allocation table (FAT); when the computer wants to store new data, it looks at the FAT map to find out which sectors are free and then moves the read/write heads to the correct location – this greatly speeds up the writing process.
Figure 3.3 Hard disk drive – the hard disk (platter) is the media and the hard disk drive (HDD) is the storage device.
While hard disk drives have much faster data access times than magnetic tape, there are still small delays. Many applications require the read/write heads to constantly seek the correct blocks of data; this means a large number of head movements. The effects of latency then become very significant. (Latency is defined as the time it takes for a specific block of data on a data track to rotate around to the read/write head). Users will sometimes notice the effect of latency, especially if many different applications are open, when they see messages such as: ‘Please wait’ or, at its worst, ‘not responding’. HDDs can be either fixed or portable.
