In addition to all that, . . .
The coercivity (sorta like magnetic sensitivity) is WRONG.
"720K" diskettes are 600 Oersted.
"1.4M" diskettes are about 750 Oersted.
It IS close (unlike 360K (300 Oersted) v 1.2M (600 Oersted)), and it WILL
work (unlike 360K v 1.2M), but it will NOT be as reliable as using the
correct diskette.
Nowadays it is much easier to get HD disks than DD ones, which is a
problem users of old machines that never had an HD drive/controller. The
think I am thining of primarily are the HP 3.5" drives (both HPIB and
HPIL models), but of course there are many other machines.
I wonder if increasign the write current would help. It should only be a
matter of changing one resistor.
-tony