This is a repeat of the information I posted in Digest 2005.11.03 which
may help you.
Formatting 1.4 Mb Floppy Diskettes as 720 kb
You can convert regular 1.44 Mb disks to 720 kb by placing a piece of
tape over the "other" hole at the top of the diskette, i.e., the one
which does not have the write-protect slide in it. This hole is used
to detect whether the disk is 720 kb or 1.44 Mb. If you cover the
hole, the drive thinks that the disk is now a 720 kb disk. A piece
of PVC electrical tape does the trick nicely.
Now you need to format the disk. Windows XP will not let you format
720 kb disks from the GUI, but you can do it from the DOS prompt. Use
the Start->Run menu item and type "cmd" and click 'OK'. This will
bring up the so-called "DOS" window of Windows XP. Then type FORMAT A:
/T:80 /N:9 and away you go! Not sure if this command-line method will
work with Windows Vista.
Covering up the detector hole is key to making the disk appear as
single-sided, not double-sided, to the drive that is reading it.