I have an Apple IIc - ROM 255 with the serial port mod (oscillator fitted).
I also have an external Drive //c. My problem is that the machine will not read from the floppy drive. I've tried swapping the drives around but no difference. I even tried plugging an Apple Disk ][ drive into the external Drive //c cable, as well as directly to the //c motherboard - same issue.
Funny thing is that it does seem to work very briefly when the machine has been left for a while and is first fired up.
I'm wondering whether this is a know problem? Could it be an issue with power supply - maybe filter caps? Or the caps in the drives perhaps?
I need to do some more testing to try further identify where the problem is - tricky one!