This was taken from an article printed in 1993. Obviously not up to date, but it answers one or two questions that nobody else appears to know!

Your first PC floppy disk was most probably five a quarter inches in diameter and had a formatted storage capacity of 360Kb. It was what is now known as a double-sided, double density diskette. Further back we have single-sided diskettes and 8 inch floppies, but we shall not be mentioning them here.

The double-sided diskette is a good place to start, because there are probably still a lot of them floating around in offices across the world. What's more, they were probably used to back-up data in the days when floppy backup was a viable solution to data security. The easiest way to identify a 360Kb floppy is to look at the centre hole. If it is reinforced with a hub ring, then the chances are that it is a 360Kb floppy, also referred to as 2S2D or 2SDD for double-sided, double density (reading the label could also give you a clue!). Modern high density diskettes are now all manufactured without a reinforcing hub ring. The ring was discarded as being unnecessary and an extra expense during manufacture.

5.25 inch Format
In 1993 this physical disk format could almost be considered obsolete. However new drives and diskettes are still manufactured and readily available. Further more, lots of users still have old information stored on five-and-a-quarters. As mentioned above, the earliest incarnation we will deal with here is the 360Kb floppy. The 360Kb floppy has 40 tracks per side, 9 sectors per track and 512 bytes per sector. These disks are highly sensitive to magnetic fields. The recording field strength of the read/write heads is 300 oersteds. Tracks are recorded using a three head write process. As the write head records the information on the disk, adjacent erase heads trim the outer edge of the track creating a very clearly defined recorded track with minimal edge fade.
Bearing this information mind, let us now consider the high-density disk formatted capacity of 1.2Mb. It has 80 tracks per side, 15 sectors per track and 512bytes per sector.
Straight away we can see that the the disk is achieving it's higher capacity by physically stacking more tracks per side and more sectors per track. More tracks can be located per side only by making the tracks thinner. Interference between adjacent tracks is overcome by making the recording material less sensitive to magnetism.
If the same material was used on 1.2Mb disks as on 360Kb disks, the tracks would interfere with one and other, gradually decaying the data stored on the disk. In order to read and write to the less sensitive media, l.2 Mb drives use a higher recording field strength of 600 oersteds.
From this very brief discussion of the differences between double and high density disks, you will see that there are mechanical and electronic differences between the two that prevent successful and reliable interchange of data between the formats. While information can be read with reasonable success from a 360Kb in a 1.2Mb drive, if you have many 360Kb disks to use, you should really be using a 360Kb drive.
If you do need to regularly use 360Kb disks in a 1.2Mb drive there are a few tips to help you in your quest. A disk written in a 360Kb drive that is then read and written to in a 1.2Mb drive will most likely work well, until returned to the 360Kb drive. Reads at this time could very well fail with the Abort, Retry, Ignore error message. This is directly related to the track recording width referred to above. 360Kb disks that must be written to in 1.2Mb drives for later use in 360Kb drives should be brand new and preferably factory formatted disks. This way the thin record track is laid down without the loud, fuzzy, pre-recorded edges of data that would already be in place had the 360Kb disk already been used in a 360Kb drive.
Disks that have been incorrectly formatted for the wrong density cannot simply be re-formatted to correct the problem either. Because formatting actually writes data to the disk, this process actually creates the problem that you may have thought reformatting would have rectified. The only solution is to completely randomise the magnetic surface with a bulk media eraser and start afresh.
5.25 inch drives do not have a media sensor to detect the type of diskette mounted. Every time you enter the FORMAT command for the drive, it attempts to format the disk at the highest capacity of the drive. While the format may succeed initially, you are in for future problems. Correct formatting can be be obtained by using switches with the FORMAT command:- e.g. FORMAT d: /4, FORMAT d: M:9 lT:40, FORMAT d: /F360 (all these commands work with MS-DOS 4.0 and latter and format the correct number of tracks and sectors with reduced write current as required by the media).

From time to time you may come across a disk labelled as "quad-density". This density rating was never used by IBM and never got the official seal of approval that has accompanied other ratings. Quad density disks use the same magnetic media and conform to the same linear data density as double density disks. If you do have any quad density disks, you can use them as good quality double density disks.

3.5 inch format
The 3.5 inch disk is actually manufactured to metric specifications and might more accurately be called a 90mm diskette. It varies in a number of ways from it's larger cousin. Firstly and most noticeably is the packaging. Although the disk media is still quite a "floppy" piece of plastic, the packaging is is quite rigid and also includes a metal sliding door to protect the read/write access opening.
Disks in the size also come in a variety of storage capacities. The most common are the double density (DD) disk of 720Kb, high density (HD) of 1.44Mb and the newest extra-high density (ED) disks or 2.88Mb. An easy way of identifying DD and HD disks is to see how many holes there are on the bottom edge of the disk. An DD just has one, the write protect hole, whereas the HD has two, the write protect hole and another hole on the other side! The problems associated with reading and writing different density disks on a particular drive are not nearly so great for 3.5in disk users. All use 80 tracks and all tracks are always the same width regardless of storage capacity. Drives of a higher capacity are perfectly capable of emulating any level of lower capacity drive.

Diskette Changeline
From AT systems on (non-360Kb drives only), pin 34 on the floppy disk data cable has been used to convey the diskette changeline signal. This signal is used to indicate whether the diskette used during the previous disk access is still mounted. If the diskette is not changed between disk accesses, the controller can speed up disk operations by using information gathered previously, thereby reducing some disk reads. The signal can be sent by a variety of methods, the most frequent of which is a switch of some kind rigged to the door latch. It operates to send the signal when the latch is opened. While this is generally a trouble free operation, there is scope for trouble with catastrophic results. If the drive does not send a disk change signal and the system thinks that the drive is other than a 360Kb, the system never recognises that the disk is changed and continues to hold the initial disk's directory list and file allocation table in a buffer. Any disk that is subseqently inserted into the drive is in grave danger of having the stored information written to it. Overwriting of the directory and FAT is as good as trashing the disk and data recovery is very difficult on a disk so trashed. If you notice that your system is displaying the directory of a previously installed disk, then you know you have the problem. Be sure to open the drive immediately to prevent writing and do not initiate any operation that would cause a write to the disk. A short term solution to this problem is to use Ctrl-C to flush the buffers each time a disk is changed. This forces the system to read the new disk data on the first access. Longer term repairs mean finding out how the signal is generated on your drive and then determine why it is not working.

Media sensor
3.5 inch, 1.44Mb and 2.88Mb drives have a sensor that enables the drive to determine what type of media is mounted and set the recording strength level accordingly. In fact 2.88Mb drives have two such sensors as the ED medium has an ID hole in a different place to regular HD disks. The media sensor is an important function if the system is to operate with as little operator concern as possible. When a properly formatted disk is inserted, the drive reads the volume boot sector to determine the format and set the appropriate mode. If the disk is not formatted the drive relies on the media sensor to set the appropriate mode to use for the FORMAT command. This can be over-ridden by command line switches following the FORMAT command.

The floppy disk controller can be either a plug in circuit board or an integrated function of the motherboard. In most cases today the floppy controller is part of a combined disk controller that also includes IDE hard disk interface and possibly even serial and parallel interfaces too. Most SCSI controllers also include a floppy interface option in the product range. The object here is to reduce the number of occupied slots in your system, which decreases power overhead for a standard computer and also gives you room to add more add-on boards when the time arises.

Floppy drives are hardy creatures and need little in the way of special care. Like all disk drives they are susceptible to wear through exposure to dust. Also, because the heads actually rub on the surface of the disk medium, they can become dirty with particles from disks over time. Occasional use of proprietary disk cleaning kits can keep this problem to a minimum. A disk cleaning kit usually consists of a special lint-free paper disk that inserts into a special jacket. The paper disk is moistened with special cleaning fluid and the whole thing mounted on a drive. Issuing any disk access command (DIR, CHKDSK) is enough to get a few revs up and allow the beads to contact the paper, cleaning off the residue. Although such commands will have the desired effect, the heads never get beyond trying to read the file allocation table causing the paper disk becomes worn in one place only. A better solution is to use a program specifically designed for disk cleaning like CLEAN.COM. This utility does nothing other than start the drive motor and move the heads in and out a couple of times. This ensures that your cleaning disk is evenly worn and used completely.
Protecting your disks in a properly designed storage box will also go along way to preserving your disk-bound data.

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Last revised: March 02, 2002