It's the disk drive not the media itself that has to be 720k. The existing drive seems to be on the blink. It's a pity but I recently chucked out an old system unit with a 720k drive in it.

Mind you I still have a 5.25 floppy reader in my current PC - I recently used it to convert a whole load of WORDSTAR documents into WORD ones on a 1.4m floppy for a friend.