Re: Apple IIc modifications

[Michael J. Mahon <mjmahon@aol.com>]

Wholly Mindless <wholly@whollymindless.com> wrote:
> Can we at least make sure that we document that for *some* people 3.5" HD
> disks DO NOT ALWAYS work?  That way when someone finds this at 3am after a
> Walgreens run and is frantically trying to format the box of 10 they bought
> and they start looking on the net, they don't assume their drive is bad and
> start tearing it apart?
> 
> My experience has definitely been down in the 1 of 10 or maybe even less
> work.  It is also possible that newer drives (or older drives for that
> matter) are more tolerant.  We just haven't done definitive research - or at
> least most of us haven't.

The difference in coercivity of HD vs. DD media is much less for 3.5"
diskettes than it is for 5.25" diskettes, so _some_ drives will be able to
write to them well enough, but HD media is outside the tolerance for DD
media, so no DD drive was designed to write it to saturation (or erase
previously written data, either). You can apparently find DD drives that
can write reliably, but that's depending on luck, not on proper design
margins. 

Proper design margins are the only thing that allows us to treat the analog
world as if it were digital. 

If the media is not written (magnetized) to saturation, then the domains
storing the data have a greater tendency to decay, resulting eventually in
unreadable data. 

Note that it's the writing that's the issue. Once a diskette is stably
written, any drive should be able to read it. 

In the case of 5.25" diskettes, the coercivity of HD media is so much
greater than DD that HD diskettes are universally unusable. 
-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon