TK> TK>> You can drill an extra hole in a 720k floppy to make it work like
TK> TK>> a 1.44M flop but the other way around (covering a hole in a 1.44M
TK> TK>> flop to make it work like a 720k) is, strangely enough, not
TK> TK>> possible.

TK> RJT> I remember trying to make one of those work,  and it wouldn't.  The
TK> RJT> extra hole isn't the only difference,  there are major differences in
TK> RJT> the magnetic stuff on the disk itself,  too.

TK>IIRC it also had something to do with a track 0 position or something...don't
TK>know exactly how or why.

Nope - see the part between the two lines of ################

Maybe this will explain some of the problems mentioned above.
Remember this was written sometime in the late 80s or early 90s.

DD and HD Floppy (Non)Interchangeability  -- by Jay
Emrie

There is a lot of misinformation and misconceptions prevalent about the
use of DD (double density) floppies (both 5 " and 3 ") in HD (high
density) drives and vice versa. A dose of facts might alleviate some ills
and at best prove to be a cure for some problems.

A floppy disk is in it's `virgin' state when it has never been formatted,
written on or, equivalently, when it has been degaussed or `bulk erased'
with a magnetic eraser, properly called a degausser.
When a floppy is formatted by the DOS format
program the data area is totally filled with Hex `F6'.
When it is formatted by other format programs the
data may be different. For example, Norton's Safe
Format (SFormat) leaves the previous data intact on
the floppy, only changing (to a special character -
Hex `E5') the first character of the file name in the
directory containing the file and removing that files entry from both
FATs. The same thing is true if you use the DOS command ERASE or
DEL to remove the files from a floppy. To DOS the file space is now
empty and can used to write other files. This is the reason Nortons and
other programs can restore SFormatted or DEL'd or ERASE'd disk files
if no writing has been done in the file area since the Sformatting or
erasure (deletion). In other words, your floppies always have some
kind of data in the data area unless they are virgin or degaussed. 

Keeping in mind the fact that data is always present, lets look at 5 ª
floppies. First notice in Figure 1 that the track width on DD floppies is
slightly more than twice as wide as the track width on HD floppies. By
doubling the number of tracks on a floppy you double the number of
bytes possible on the floppy. Further, by almost doubling the data
density in the tracks, you enable a HD floppy to hold almost 4 times as
much data (1.2 MB compared to 360 KB) as a DD floppy. In order to
be able to do this, the data surface must be different; the magnetic
particles are much finer, making the coercivity of the HD floppy surface
two times that of the DD floppy. This increased coercivity requires a
much stronger write current passing through the write heads, thus
imparting a stronger magnetic force on the data surface. If a 360 KB
floppy is written on in 1.2 MB mode the magnetism imparted is far
greater than it should be. This is much the same as recording upon
audio tape at twice the proper signal strength. The data bits on the
floppy surface will be distorted much the same as the audio sound is
distorted on the audio tape under the excessive signal conditions. This
is further complicated by the fact that the overly strong adjacent
magnetic fields tend to merge together with time, thus corrupting the
data even more, increasing the probability of data loss. Once a 360 KB
DD floppy has been written on by a 1.2 MB HD drive in HD mode the
floppy is not reliably usable in any drive until it has been degaussed.
By degaussing you remove all magnetism put on by the excessive
write current. Just writing over this with a DD drive or a HD drive in
DD mode WILL NOT get rid of the excessive magnetism! Degaussing
is the only solution. FYI, a magnet only magnetizes the surface - it
does not remove all magnetism, but will cause data checks. The above
completely disproves the misconception that the difference between
DD and HD disks is just a more stringent testing by the maker. It is
actually a different magnetic recording surface, formulated to
accommodate the different data densities.

Another complication arises because of the different track widths
written by DD (360 KB) and HD (1.2 MB) drives. The DD track width
is 0.330 mm wide and the HD track width is only 0.160 mm wide. If the
floppy was ever formatted by a DD drive and then overwritten by a HD
drive the result is a 0.160 mm HD track in the center of a 0.330mm DD
track leaving two residual DD 0.085 tracks, one on each side of the
0.160 HD data track. Now if you try to read this track with a DD head
(which is wider than the HD head) the DD head will read the narrow
HD track plus the residual DD tracks on each side of the HD track. The
left over DD tracks can cause read errors. 

Have you ever been given a floppy and had read errors, retried reading
several times, maybe succeeding with a correct read after a few retries?
Begin to smell a rat or two?

If one must use a DD 5" floppy in a HD drive, use a new unformatted
floppy, formatting it in the HD drive and then writing on it with the HD
(Continued to next message)

---
 þ OLXWin 1.00a þ An appeaser feeds the crocodile, hoping to be eaten last.