The 3.5 inch diskette, devised by Sony, contains a non-rigid magnetic disk with
80 tracks on each side. There are 135 tracks per inch (tpi), giving a data density of
around 5000 bits per inch (bpi). The disk rotates at about 360 revolutions per minute
(rev/min) and transfers data at between 200 and 400 K a second.
Diskettes and Drives
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Capacity
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A diskette’s data capacity is determined by the recording method and the disk itself.
Standard formats include:-
Single Sided SS 400 K
Double Sided DS 720 K
Double Sided DS 800 K
High Density HD 1.4 M
Extended Density ED 2.8 M
Most modern diskettes are of the DS or HD variety — the FDHD or SuperDrive mechanism inside modern Macs accepts any types of diskette except the ED version. The original drives used an Integrated Wozniak Machine as a controller — later drives contain a Super Integrated Wozniak Machine (SWIM)!
DS diskettes, containing 9 sectors per track, hold exactly 800 K, that’s 819,200 bytes in total — including the space used for invisible files such as the Desktop file.
The HD version uses 15 sectors per track and holds 1,474,560 bytes, including invisibles.
Compatibility
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All HD diskettes have an HD sensing hole, also known as the bonus hole, to the left of the label. But a drive designed only for SS and DS diskettes doesn’t respond to this hole — it always formats an HD diskette to the DS standard, reducing its capacity to 720 or 800 K.
If you insert such a wrongly-formatted HD diskette into an FDHD or SuperDrive mechanism it senses the bonus hole and goes into HD mode. Since the disk is actually formatted to the DS standard, it can’t read the diskette until you put tape over the hole!
Once formatted this way an HD diskette can never be reused at its full 1.4 M capacity. This is due to the fact that DS diskettes are recorded with a higher magnetic field that can’t be erased using a modern drive. Newer Macs, equipped with a SuperDrive mechanism, will automatically format an HD diskette correctly — assuming it’s not got tape over the hole!
fl 400 K or 800 K diskettes are formatted using the Mac’s own GCR system (see below).
These disks can’t be read by other computers unless specially designed for them.
However, with PC Exchange and a SuperDrive mechanism, you can format 1.4 M disks
for the Lisa 2, MS-DOS, OS/2, PC-DOS and ProDOS (for Apple II) operating systems.
fl Extended Density (ED) disks provide 2.8 M of storage but are not available for the Mac.
Such drives can also read standard or HD disks. ED disks, identified by an ED logo and
an inland sensing hole, are not suitable for any other drive.
Diskette Hints
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DON’T:-
  Bend disks
Touch the magnetic material
Stack objects on top of disks
Use erasers on the label
Press too hard when writing on the label
Pile too many labels on top of each other
Stick your disks to the filing cabinet with magnets
Use abrasive disk cleaners
Clean your drive more than once every 6 months
Tamper with your drive
Erase a diskette unless you’re certain — data can’t be recovered!
DO:-
  Keep floppy diskettes away from magnetic fields
Beware of metal detectors
Avoid smoking or eating whilst handling disks
Use a drive cleaner recommended by Apple, such as Kodak or 3M
When cleaning use a utility to rotate the disk for up to 20 seconds
Employ a utility to test the heads
Return backup disks to archive after 6 months to reduce wear
fl Diskettes can’t be damaged by normal heat, static electricity, radar, microwave
transmissions or X-rays. The magnetic fields in X-ray machines may cause damage!
fl The yellow transit disk that comes with older Macs isn’t needed for FDHD or
SuperDrive mechanisms with self-parking heads — but using it can’t do any harm!
Loading and Ejecting
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If your Mac refuses to accept a diskette you could try:-
• Pressing Option-† whilst inserting the diskette — this rebuilds the desktop.
• Putting the diskette in another drive.
• Rotating the floppy disk by hand and then reinserting it.
A diskette that refuse to work in a Mac may be used in a PC — or you could try to format it as a 720 K Mac disk (not 800 K) with special software such as Disk Charmer.
fl To eject a diskette you can press Shift-†-1 or Shift-†-E for the first drive or Shift-†-2
for a second drive — but this doesn’t remove its icon from the desktop in the same
way as trashing it. To remove it, select the icon and choose Put Away or press †-Y.
fl A flashing X icon at startup may indicate a diskette is jammed in the mechanism!
Release it by pushing a straightened paper clip into the small hole next to the drive.
fl PaperClip (Ben Hellstrom) is a useful utility that can eject a diskette that’s stuck in a
drive for software reasons — see the previous entry for how it got its name!
Salvaging Damaged Disks
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To rescue a diskette that has fallen into liquid (coffee, for example) first remove the magnetic disk itself from the case by breaking away the corners opposite the shutter with a thin knife. The disk can be washed under the tap (yes, really) and fitted into a new case.
Believe it or not, it’s possible to rescue data from a terribly damaged diskette. For example, if the actual magnetic disk has been crumpled up it can be ironed flat, or if it‘s been cut up into slices it can be repaired with 3M tape — and 80% of the data recovered!
If a diskette has a bent shutter this should be carefully removed. Its contents can then be copied onto another diskette before disposing of the original rogue disk.
fl Always avoid touching the magnetic surface of the floppy disk itself.
Drive Mechanisms
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A floppy drive mechanism doesn’t usually require any attention. It’s probably best to call in the experts if you have problems!
If there are ejection problems a small amount of lubricant may be applied to the side rails. Skilful individuals may be brave enough to replace faulty micro-switches in older drives.
The head alignment on most drives is adjusted by means of a black screw located adjacent to the gearing. This is not for the inexperienced and certainly not for those without an alignment disk — unless you’re very lucky!
Diskettes in Detail
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Formatting
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When you put a diskette into your Mac it looks for formatting information. Early drives only accept diskettes formatted to the Mac SS or DS standards. An FDHD or SuperDrive mechanism also takes HD floppies — with PC Exchange it also accepts Lisa 2, MS-DOS, OS/2, PC-DOS and ProDOS (for Apple II) diskettes.
If your Mac doesn’t see any formatting it assumes the diskette is unformatted (or faulty) and presents you with the Erase Disk… window. This shows a menu of available formats — what you see depends on your drive and whether PC Exchange is installed.
If you’ve decided to format the diskette the process begins with initialisation, in which the sectors are divided into logical compartments or blocks. The Mac then checks to see if there are any bad sectors and finally creates a disk catalogue that’s used to locate a file’s contents by means of its sector address. The first two blocks, blocks 0 and 1 contain the boot blocks whilst block 2 contains the actual directory.
For batch formatting, or where special options are required, you should use a disk formatting utility. One example, Disk Charmer, can wipe a disk by erasing the disk catalogue or verify it for data integrity. It can also make larger disks, using space normally avoided by Apple, and even lets you use bad sectors! And it can create 720 K Mac disks — useful for those old disks you can’t format any other way!
fl The earliest Macs used 400 K single sided disks and the Macintosh Filing System (MFS).
This is different to the Hierarchical Filing System (HFS) in modern Macs and isn’t
supported by System 7. You can use Disk Charmer to format a MFS disk.
fl Some older disks designed for a track density of only 67.5 tpi can give problems!
Formatting may use either Group Code Recording (GCR) or Modified Frequency Modulation (MFM). Older Mac drives used GCR but the modern FDHD and SuperDrive varieties also accommodate MFM, used in PCs and other devices.
Group Code Recording
GCR, a system exclusive to Macintosh, uses a sector size fixed at 512 bytes for all tracks. Hence the number of sectors in a track varies continuously across the disk. For example, the 16 outermost tracks have 12 sectors whilst the innermost have only 8. This ensures a constant data density that makes the disk equally reliable throughout. To make this possible the speed of the disk must be adjusted as the read/write head moves across it — the drive motor runs slowly whilst using outer tracks and speeds up for the inner tracks.
Each sector also contains tag data, as used in older drives for data verification and file recovery. Tag data includes information about each file number, its resource or data fork and the modification date. Due to of cost, tags were abandoned with the introduction of the double-sided drive. The Lisa computer’s OS won’t even work without tags!
Disk capacity is 400 K (single sided) or 800 K (double sided).
fl Macs often produce a GCR disk automatically if you select Erase Disk… in the Special
menu. If PC Exchange is enabled just select Macintosh 400K or Macintosh 800K.
Modified Frequency Modulation
In MFM disks the sector size and data density varies across the disk — the drive motor runs at a fixed speed. This system is widely used by non-Mac computers and other devices.
Disk capacity is 720 K (double sided) or 1440 K (double sided, high density).
fl To produce a MFM disk you must first ensure that PC Exchange is in operation.
Then select Erase Disk…in the Special menu and choose DOS 720K or DOS 1.4MB.
Disk Sizes
The standard sizes are:-
Computer Size (K)
Mac SS 400
Mac DS 800
Mac HD 1440
Lisa SS 400
Other SS GCR 400
Apple II Pascal 800
Apple II ProDOS 800
Other DS GCR 800
Mac A/UX 800
MS-DOS DS 720
Other DS MFM 720
MS-DOS HD 1440
Other HD MFM 1440
fl Once you’ve formatted a disk you’ll want to name it! Unlike other Mac icons the
names of disks are limited to just 27 characters.
Mounting
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The Mac uses specific drive numbers, such as 1 for the internal floppy and 2 for the external drive. These numbers don’t change even if a drive isn’t used. These numbers appear in keyboard shortcuts used for ejecting diskettes, such as Shift-†-1 and Shift-†-2. Other computers such as PCs use drive identifiers such as A: and C:.
When you first insert a diskette, your Mac gives it a Volume Reference Number (VRN) and creates a Volume Record for information about the disk. The floppy is mounted, is on-line and appears on your desktop.
If you Put Away or trash a floppy, it’s unmounted, it’s icon disappears and all is forgotten, including the VRN which is now available to other drives. But if you choose Eject Disk, the icon stays visible and turns grey. The Mac still remembers the VRN but not the Volume Record — the diskette is mounted and off-line. If any application needs it, the Mac asks you to insert the diskette which then returns to the on-line condition.
fl A useful extension called Real Eject forces your Macintosh to Put Away a diskette
whenever you select Eject Disk or press †-E. This avoids those irritating grey icons!
Copying
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Copying Software
Several applications can copy all kinds of diskettes, whether your machine has one drive or more. Examples include Disk Copy, DiskDup and DART.
Most of these applications can store replicas of diskettes as image files on hard disk, often compressed with a fast algorithm, such as run-length encoding (RLE). The Lempel-Ziv algorithm with Huffman adaptive encoding (LZH) is slower but more effective.
Some applications also let you mount disk images as you would real diskettes. You can drag items out of these files without fear of damaging the original data.
Copy Protection
There are various ways of preventing illicit copying. The most effective methods modify low-level information on those parts of a diskette only used by the System and internal disk drivers. This causes read errors when a diskette copying program is used.
More common forms of protection include the use of an Installer application that lets you make a specific number of copies from floppy onto hard disk(s). You may be able to de-install some copies but the Installer remembers the exact location of copies on the drives. If you try to make further installations you’ll be thwarted! More limited forms of protection use a password — this can’t really stop illicit copying of registered versions.
Other Floppy Disk Formats
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Earlier machines, particularly PCs, use the 5.25 inch format instead of diskettes. These have a capacity of 360 K or 1.2 M, at a density of 48 and 96 tpi (9 and 15 sectors per track) respectively. Strictly speaking, 360 K disks shouldn’t be used in a 1.2 M drive — they use different recording levels and materials. In practice they often work, especially if the disks aren’t stored for a long period.
