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You've gotten a good bunch of replies.  More comments:

In article <chris.3717@genly.UUCP> chris@genly.UUCP (Chris Hind Genly) writes:
 > The two types of floppies appear identical to the eye. 

That's not quite true, at least not with the diskettes which I have
experience with (specifically, most of them are either brand name or
from the mail order company MEI).  If you hold them up so that light
glares across the disk surface, you can see that the high density disks
have a bluish tinge and are a lighter color than the brownish
double-density floppies.

I would disagree with <rschmidt> in that a HD should be able to be
formatted as a DD.  Since the two kinds of disks are made with different
materials with different magnetic coercivities, you can neither format a
high-density to low-density nor low-density to high density.  The
low-density drive is *NOT* indifferent to the medium.  Like you, I have
had no success getting my (Teac) low-density drive to format
high-density medium.  Also, I think a high-density drive uses a more
powerful but also more focused magnetic flux in its heads, not less.

<cctr132> is incorrect about HD diskettes being preformatted, because
they are not, unless you pay through the nose for the special ones which
have been.  The DOS-FORMAT program as a general rule does not check the
disk to see what density it is; it only checks the BIOS for the drive
type and any relevant command line switches.  (This can be verified with
one of several TSR DOS function snoopers, which show DOS calling the
BIOS format disk option with the appropriate number of sectors for the
format selected without ever reading from the disk.)  Many third-party
replacements for FORMAT.COM, such as the ones which come with Norton or
with PC-Tools, have a quick-format option which does always format a
floppy which already been previously formatted to the same capacity.
With this option, they do not do true formatting at all, but merely wipe
the directory and the FAT tables for the diskette.

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