I have a Macally USB floppy drive which I use on both my Bondi iMac and Flat Panel iMac. I can read/write DS/DD (726,016) floppy disks formatted from the Atari (GEM) Desktop without any problem at all. I've never tried SS/DD disks.
I can also format a DS/DD DOS disk on my Mac using the USB floppy drive and read and write to it on my real Atari STe.