MOUNT won't work

ok, first of all you don't need to MOUNT a D81 if you already attached it to XEMU (Rightmousebutton -> Disks -> Drive-8 -> Attach D81) because that one will always be #8 ... just do a DIR and you will see the directory.

MOUNT would be used, if you want to mount a D81 on your (virtual) SD card. Something like MOUNT"MEGA65.D81" and the D81 from your SD card will be mounted as Drive#8