It sounds like something is weird with your floppy drive. You said you already replaced it? Make sure the drive is jumpered correctly and also make sure it's connected to the correct part of the floppy cable (before the twist, if I remember the user's manual correctly).
If that doesn't work, you may be able to reset the drive manually after swapping disks with a DISKCHANGE DF0: command in the shell. That may only work for 5.25" floppies, though, I don't remember if it's valid for standard 880K ones.
