On 4/2/2009 Adrian Thomas wrote:

    So a recent (well, since the late '80s) DD disk is just
    a slightly dodgy HD disk in a case without the extra hole.

Sorry for the long post, but there have been enough posts on the
subject to suggest that a comprehensive and correct procedure should be
outlined.

Mr. Thomas is correct.  The compatibility problem is with the strength
of the recording.  An HD (1.44 MB) disk is recorded using a stronger
magnetic field.  This leaves a stronger magnetic signature on the disk
which cannot be effectively erased by the DD (720) recording strength
during a reformat.

All magnetic information can be erased completely by use of a video
tape bulk eraser, and then the disk can be safely formatted using the
correct 720 command line switches in the format command.  To use a
bulk eraser, remove your watch, hold the disk and eraser about three
feet apart and press the power trigger, then bring the eraser to the
disk and in contact with the jacket, moving it in slow circles for five
or six seconds, and then separate the eraser and disk by three feet
before shutting off the eraser.

It doesn't hurt to repeat the process for the other side, although it
is probably not necessary.  If you are erasing a stack of disks, your
eraser will probably shut down before completing the work.  Erasers
have thermal switches to turn them off when they get hot.  The duty
cycle is usually about two minutes per hour.  A video tape eraser is
needed; a pencil-type tape head eraser cannot do this job.

You cannot rely on the missing hardware hole to tell the operating
system that you have a 720 certified disk in the drive.  Use the
command line switches to force a 720 format.  On most computers and
operating systems the hole is ignored.  On older operating systems,
Windows 95 & 98 for instance, the disk will be formatted for the
highest density (HD in most cases) that the drive is capable of,
regardless of whether there are two holes in the jacket.

A drive that has not been used for several years will have a build-up
of grime that must be worn off before you can use it reliably.  At one
time wet-dry cleaning disks were made for this purpose, and they work
well if you can find one.  Today I would recommend that you use a
sacrificial disk for this purpose.  Format it several times to give
the drive some exercise.  Put an X on it with a magic marker to let
yourself know that this disk should not be used for data.

When reformatting disks, include the command line parameter /U along
with the appropriate 720 format switches to force an unconditional
format, which will re-write the entire disk completely.  Otherwise only
a few sections of the disk are re-written and the data areas and all
sector and track headers are retained from the previous formatting
procedure.  It is best to start with everything fresh and magnetically
sound.  Even preformatted disks that have never been used should be
freshly formatted, especially if they are twenty years old.

You must specify your floppy disk drive explicitly; otherwise the
format command will default to the drive you are presently logged
onto--usually your computer's hard drive.  Usually you will get a
warning before this irreversible command is executed, and you can back
out of it.  But be alert to "Ready to format fixed disc...press any
key to continue," as it will do just that if you press any key.
Instead, take your hands off the keyboard, read it three times, utter
your favorite cuss word, and pull the plug.

Assuming your 3 1/2" drive is Drive A, the following is a valid
command in a DOS window and will format a 720 disk unconditionally in
Drive A:

     format a: /n:9 /t:80 /u

As shown below from my DOS window:

    C:\>format a: /n:9 /t:80 /u
    Insert new diskette for drive A:
    and press ENTER when ready...

    Formatting 720K
    Format complete.

    Volume label (11 characters, ENTER for none)?

    730,112 bytes total disk space
    730,112 bytes available on disk

    1,024 bytes in each allocation unit.
    713 allocation units available on disk.

    Volume Serial Number is 0E13-13F7

    Format another (Y/N)? n

If there are bad sectors on the disk, the report will say so and show
the number of bytes available on the disk to be smaller.  I recommend
that, if there are bad sectors, you discard the disk or erase it with a
bulk eraser and try again.  When 720 KB disks were in use, there were
many problems with disk quality and a number of brands frequently would
not format without resulting in bad sectors.  Such disks frequently did
not retain data as long as six months.

3 1/2" drives are available from Newegg, Bestbuy, and other places,
that will connect to a USB port.  Some people report problems using
these drives with Windows Vista; others say it works fine.

Don Shenbarger