The A2000 with board part number 312723 has jumper J301 This jumper is closed to add a second internal floppy drive, open to leave the second floppy out of the main unit box. The X X Amiga expects an ID bit stream from each floppy drive; this lets it determine the drive type. External floppies have this ID circuitry on board, but as it's not an industry standard capability, it has to be implemented on the A2000 motherboard to save cost on internal drives. Leaving the jumper open prevents the Amiga from seeing the ID sequence.
A2000 with board part number 380705-01 - jumper J36.