don't use AT-compatible floppies in a 3B2!!!

[Bill Mayhew]

High density (like "AT" 1.2 megabyte disks) are made from a
different sort of magnetic material, similar to chromium dioxide
cassette tape.  If you look at the surface of an HD disk it looks
sort of black, while standard floppies have a brownish color.  The
material in HD disks has a larger valued coercivity, which means
that it is more difficult to (re)magnetize.  Regular disk drives',
even DSDD drives', recording heads don't putout enough magnetic
field to properly write the HD medium.

The reason for failure, in this case, is not related to bit
density.

Ironically (iron .. a pun), one is more likely to have success
attempting to format a regular disk in an HD drive than vice -
versa.  I would not reccomend it, however.

--Bill