If either or both of these are correct, disconnecting the card reader unit in the floppy drive might resolve the issues (the reader slots in the all-in-one are much easier to reach in any case). This leads to the question of whether it's worth bothering to buy and install a plain floppy drive; I can't recall the last time I used a floppy (including the one time my system had to be steam-cleaned and everything reinstalled from scratch and restored from backup for reasons unrelated to the above).
That raises my curiosity: How long has it been since anybody else actually used a floppy disk?
(I ended up disconnecting the MP830 and connecting its USB cable to the external drive to perform the backup, with no problems except the inconvenience. I may need to pick up a USB hub, if I can find room on my desk.)