To read and format disks with formats other than 720kB (the
mformat utility refers to this as 708K) on Windows NT you will need the [http://simonowen.com/fdrawcmd/ FDrawcmd driver by simon owen]. This will only work correctly on PCs with real floppy controllers.
A good utility for writing the images is [http://www.seasip.info/Unix/LibDsk/#tools John Elliot's dsktool].
It's possible to read and write 708K +3DOS format dsk images using dsktool on some USB floppy drives that have an NEC UF000x chipset. The Dell FDDM-101 module is known to work.