Re: HD floppies in a DD drive.

[Michael Mahon <mahon@cup.hp.com>]

Michael Ewen wrote:
> 
>         Thanks for all the advice.  Since the major use I have for 3.5
> disks is as backups for my hard drive I think that I'll look for some DD
> disks.
> 

Hey, you sure got a ton of advice, all right!  On the 'net everyone
has an opinion!

As some of you pointed out, however, there is a little tiny *fact*
bouncing around here:  HD disk magnetic media is *not* the same as
DD media.  At least, not if the manufacturer is abiding by the standards
for making the disks...   ;-(

HD media is "high coercivity" magnetic material (like, as someone
pointed out, high-bias audio tape), and is designed to be written at
a higher magnetic field level than the design center for a DD drive.

Your *particular* DD drive may write a little "harder" than the DD
standard requires; and that *particular* HD disk you're writing may
have a slightly lower coercivity coating than the design center for
HD disks.  So you *may* be able to write reliably on the HD disk...

On the other hand, neither the drive designer nor the disk media
designer was planning to make that combination work reliably, so you
may *actually* be writing barely-strong-enough magnetic transitions
on the HD disk, which, given the normal slow decay as time passes,
may eventually no longer produce a reliable read signal on your DD
drive.  Result:  data loss followed by sleep loss or (even) hair loss!

This isn't just a little "fudge", like notching and flipping your
floppies, which works realiably if you are tidy; this is using a
flat screwdriver to operate a Phillips screw--it can work, and it
may work for you, but it wasn't the plan, and it's not guaranteed.
When you use an HD disk in a DD drive (or _vice versa_, but that's
a slightly different story), you're choosing to operate with little
or no margin.  And MARGIN is the only thing that allows us to pretend
that all this analog stuff is digital!  ;-)

-- 
-michael

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This is my personal opinion, and does not in any way represent
the position of my employer, etc...