Floppy Disks.

Diskettes, often called floppy disks, are single platter plastic disks in a plastic jacket. The plastic jacket is quite hard on modern 3½" diskettes, but they are nevertheless still classified as floppy disks, not hard disks as some people think.

Floppy disks are characterised by several parameters:

Size, meaning the diameter of the disk.

Because floppy disks were developed in the USA, the sizes are still in imperial measurements. The first floppy disks were 8". They are not in use any more.

The next size was 5¼". These disks are getting out of date and used mainly in older systems.

The standard size of floppy disks today is 3.5".

Single or double-sided.

Single-sided disks are not in use any more. Floppy drives have two read/write heads – one for each surface.

Density is the number of bytes that can be stored on each track.

As diskettes were developed there have been changes in the density. Originally on DOS computers the density was 1.25 kilobytes per inch of track. As disk drive and disk technologies were improved disks that could hold 2.5 kilobytes per inch were released and called (logically) double density disks. Later again further improvements permitted quad density, now commonly called high density disks and drives. The original low density disks have now disappeared, leaving only double and high density disks. This results in some confusion for new users as the name double density seems to imply it holds more than the other current type - high density.

Double density diskettes are no longer common. They are generally labelled DD or 2D. In addition, on 5¼" disks they have a ring around the hub and on 3½" disks the hole near the bottom right corner is filled in.

High density disks are usually labelled HD. 3.5" high density disks can hold up to 1.44M of data.

In the past couple of years the density was increased and disks that can hold 2.88M appeared on the market but they are not widely used.

The details about the various types of floppy disks used on DOS/Windows computers are summarised in the table below:

The total capacity of a disk is a multiplication of surfaces by tracks by sectors. So, a 3½" high density disk can hold 2 x 80 x 18 x 512 = 1,474,560 bytes (1,024 = 1,440K).

Densities are an issue only when using older computers that have double density disk drives. High density disk drives will read and use any density disk. However, computers sold before about 1990 often had double density disk drives and these cannot read high density diskettes.

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Diskettes rotate within their permanent jacket at either 300 or 600rpm. The jacket is lined with a fabric to clean the disk as it spins and to prevent the friction of plastic rubbing against plastic.

Access slots for read/write heads are located on either side and allow touching the surfaces. On 5¼" disks the slots are protected when the disk is not in use by a paper envelope. On 3½" disks a spring-loaded metal shutter protects the access slots (the disk drive opens this shutter automatically as the disk is inserted).

All disk drives have indicator lights that show when the disk is actually being used. A diskette should never be removed while this light is on. You should always glance at this light before removing a diskette.

Floppy disks, like audio and video cassettes, can be delete protected. On diskettes it is called write protection. It protects data from deletion and alteration. If the diskette is never to be altered or contains important data this is a useful facility. If you are attempting to save onto a diskette and DOS responds with the message Write Protect Error, this indicates the diskette has been write protected. To write protect a 3½" disk, the notch on the bottom left corner should be opened. On 5¼" disks, write protecting is done by covering the notch in the top right corner with a silver tab (which is provided when a box of diskettes are purchased). To remove write protection you reverse these actions.

Advantages of using floppy disks:

they allow to copy files from one computer to another;
they are quite cheap.

Disadvantages of using floppy disks:

they have a limited capacity (graphics files often don’t fit on a disk);
they are relatively slow;
they are less reliable than hard disks so proper care should be taken to prevent loss of data.

It is important to take proper care of your disks to minimize the danger of losing data.