The 3½ inch micro diskette or floppy disk, as employed in many computers, was originally devised by the Sony Corporation. Unfortunately, this kind of medium has inadequate capacity for modern documents and applications, although compatible drives such as the SuperDisk have extended the format for a short time. Because of these limitations, diskette drives aren’t fitted inside some modern machines, although an external floppy drive can be connected via a spare USB port.

Basic Principles

Within the outer shell of a diskette there’s a non-rigid magnetically-coated disk, usually divided into 80 tracks on each side. Modern disks usually have 135 tracks per inch (tpi), giving a data density of 5000 bits per inch (bpi). A standard disk rotates at around 360 revolutions per minute (rev/min), giving a data transfer rate of between 200 and 400 KB per second (KB/s). The head, positioned beneath the diskette in a single-sided drive, reads data from a zone approximately 20 mm wide.

Diskettes and Drives

A diskette’s data capacity is determined by the recording method that’s used for storing data and the nature of the disk itself. The most common types of diskette include:-

Single-​sidedSSMac400 KB
Double-​sidedDSPC720 KB
Double-​sidedDSMac800 KB
High ​DensityHDMac/​PC1440 KB
Extended ​DensityEDPC2880 KB
HiFD Mac/​PC100/​200 MB
SuperDisk ​LS-120 Mac/​PC120 MB
SuperDisk ​LS-240 Mac/​PC240 MB

The capacities shown above include the space used for formatting, which is of course essential. The actual capacity of most disks prior to formatting is given below:-

Single-sidedSS500 KB
Double-sidedDS1 MB
Double-sidedDS1 MB
High DensityHD2 MB
Extended DensityED4 MB

Traditionally, Macs use double-sided (DS) or high density (HD) diskettes, although older models don’t accept HD diskettes and can’t format PC-compatible 800 KB floppies. Fortunately, later models have an FDHD or SuperDrive mechanism that accepts most types, although advanced variations, such as ED, HiFD or SuperDisk, always require a special drive.

The most common formats are reviewed below. The given storage capacities include the space required by the Mac’s invisible desktop files but exclude the capacity consumed by formatting.

A floppy disk drive can also be used to read data from any kind of SmartMedia card, as used for storing images generated by a digital camera. To do this, you simply insert the card into a FlashPath floppy disk adaptor and then insert the adaptor into a diskette drive. At the time of writing it appears that this kind of adaptor can’t be used in a SuperDisk drive.

Filing Systems

Both HD and SuperDisk diskettes can be formatted on a Mac OS machine or on a PC. However, these computers use entirely different filing systems, meaning that a PC can’t normally read Mac-formatted HD diskettes. Fortunately, this isn’t usually a problem, since the modern Mac OS can read or format any type of diskette.

Diskette Compatibility

HD diskettes have an HD sensing hole, also known as a bonus hole, on the opposite edge to the write-protect hole. Unfortunately, older drives designed for SS and DS diskettes don’t sense this hole, so they format all HD diskettes to the DS standard, reducing the capacity to 720 or 800 KB. Once formatted this way, the original 1440 KB capacity of an HD diskette is lost forever. This is because DS formatting employs a higher magnetic field that modern drives simply can’t erase.

The bonus hole on such a wrongly-formatted HD diskette makes an FDHD or SuperDrive drive go into HD mode. However, since the diskette is actually DS formatted, the drive can’t read it unless you cover the hole with tape.

Diskette Hints




Solving Problems

Loading and Ejecting

If a computer fails to accept a diskette you could try:-

A diskette that’s refused by a Mac OS computer may work perfectly in a PC. Alternatively, you could format it as a 720 KB Mac OS disk (not 800 KB) by employing special software, such as Disk Charmer for the Classic Mac OS.

Damaged Disks

To rescue a diskette that’s fallen into liquid, such as coffee, first remove the magnetic disk itself from the case by breaking away the corners opposite the shutter with a thin knife. The disk can then be washed under the tap (yes, really) and put in a new case. Believe it or not, you can rescue data from a terribly damaged diskette. For example, if the magnetic disk is crumpled it can be ironed flat or if cut up into slices it can be repaired with 3M tape; and 80% of the data can be recovered.

Drive Mechanisms

A floppy drive mechanism doesn’t normally need any attention; and it’s probably best to call the experts if you have any problems. However, if you have trouble with ejecting diskettes, a small amount of lubricant applied to the side rails may fix it. If you’re skilful, brave or just plain reckless, you can also replace a microswitch, should one fail in an older drive.

The head alignment on most drives is usually adjusted by means of a black screw located adjacent to the gearing. Tinkering with this isn’t for the inexperienced or for those without an alignment disk, unless of course you happen to be very lucky!

Technical Details


When you insert a diskette, the computer looks at the formatting. Very old Mac OS drives only accept Mac SS or DS formatting, whilst an FDHD drive or a SuperDrive also identifies HD floppies. Modern drives also accept Lisa 2, MS-DOS, OS/2, PC-DOS, ProDOS (Apple II) and other formats, but only with File Exchange or PC Exchange in the Classic Mac OS. If your computer doesn’t see any formatting, it assumes that the diskette is unformatted or faulty.

Formatting begins with initialisation, in which the diskette’s sectors are divided into logical compartments or blocks. Checks are then made for any bad sectors and a disk catalogue is created, which locates the content of each file using sector addresses. The first two blocks (blocks 0 and 1) contain the boot blocks whilst block 2 contains the actual directory.

A disk formatting utility can be used for batch formatting of diskettes or where you have special requirements. Disk Charmer, which works in the Classic Mac OS, can quickly wipe a diskette by removing its catalogue or can verify a disk’s integrity. It can also make larger diskettes, using space normally avoided by Apple, and even lets you use bad sectors. Finally, it can create 720 KB Mac OS diskettes: useful for those old floppies you can’t format any other way!

Older Mac OS drives employ group code recording (GCR) formatting, whilst the later FDHD and SuperDrive disk mechanisms also accommodate modified frequency modulation (MFM), which is also used in PCs and other machines.

Group Code Recording (GCR)

The Mac’s exclusive GCR system uses a fixed sector size of 512 bytes for all tracks, meaning that the number of sectors in a track varies across the disk. Hence the 16 outermost tracks have 12 sectors, but the innermost ones only have 8. This gives a constant data density, making the disk equally reliable throughout. However, the speed of the disk must be adjusted as the read/write head moves across it. Hence it turns slowly whilst using outer tracks and speeds up for the inner tracks.

Each sector also contains tag data, used in older drives for data verification and file recovery. This includes information about each file number, its resource or data fork and the modification date. Due to the extra cost, tags were abandoned with the arrival of double-sided drives, although the Lisa computer system doesn’t even work if tags aren’t present.

Modified Frequency Modulation (MFM)

In a MFM disk the sector size and data density varies across the diameter of the disk, with the drive motor running at a fixed speed. A 720 KB disk has 12 sectors per track whilst the 1440 KB version uses 18 sectors per track. This system is widely used in many non-Mac OS computers and other processor-based devices.

Disk Sizes

The standard disk sizes are summarised in the following table:-

ComputerSize (KB)
Mac SS400
Mac DS800
Mac HD1440
Lisa SS400
Other SS GCR400
Apple II Pascal800
Apple II ProDOS800
Other DS GCR800
Mac A/UX800
Other DS MFM720
Other HD MFM1440


The Mac OS uses specific drive numbers, such as 1 for the internal diskette drive and 2 for an external drive. These numbers, referred to in Classic Mac OS shortcuts, such as -Shift-1 and -Shift-2, don’t change even if a drive isn’t used. Other types of computer, such as a PC, use alternative drive identifiers, the most common of these being A: and C:.

When you first insert a diskette, the computer gives it a volume reference number (VRN) and creates a volume record for the disk information. The floppy is mounted, is on-line and appears on the desktop. If you choose Put Away or put it in the Trash, it’s unmounted, the icon disappears and all is forgotten, including the VRN which is then available to other drives.

Other Floppy Disk Formats

Early PCs use 5¼ inch standard diskettes of various types, with a capacity of between 160 KB and 1.2 MB, although the single-density variety was obsolete even before the PC appeared. The standard double-density version uses 40 tracks at 48 tpi whilst the quad-density and high-capacity types, the latter introduced with the IBM AT PC, have 80 tracks at 96 tpi.

Each sector contains between 128 and 1024 bytes, although most diskettes use 512 bytes. Double-density and quad-density diskettes have 8 or 9 sectors per track whilst the high-capacity type has 15. Early versions of the Microsoft Disk Operating System (MS-DOS) for the PC can’t accommodate all the common disk types, as shown below:-

DensityTSBytes/​sectorSize ​(KB)DOS

where T is the number of tracks and S is the number of sectors

Note that the original version 1.0 of MS-DOS doesn’t recognise a double-sided (DS) disk, although it does accept the single-sided (SS) variety. A SS disk isn’t designed to work in a DS drive but can often be formatted as a DS disk that works perfectly normally, although there’s no guarantee of this happening with every disk.

The capacities shown in the above table don’t include the space for formatting, which is of course essential. For example a 360 KB disk has a total capacity of 500 KB, including the formatting, whilst a 1.2 MB disk has a total capacity of 1.5 MB.

Fitting Disk Drives in a PC

Floppy disk drives are usually connected inside a PC via a special 34-way ribbon connector. There are actually three connectors on this ribbon: one plugs into a floppy disk controller card whilst the other two can connect a pair of drives. Power connections are usually the same as those used for standard disk drives: either a full-sized 4-way Molex connector or a miniature equivalent. You’ll also need a power splitter cable if there aren’t enough power connectors in your machine.

Kits can be purchased, allowing you to install a 3½ inch drive into a 5¼ inch slot. This usually includes a suitable type of faceplate, although you may also need to obtain special mounting rails.

©Ray White 2004.