the old drives use 12V power too which USB has not. There are dapter kits that contain a power supply and various plugs for HDD and floppy:

This one is just like the one I have and it works OK to read out old disks.

There are USB floppy drives available at the same price. 20 australian $ seems OK.

Depending on the vintage of desktops, chances are you find one that has the right plugs inside for any older floppy drive.