After some mails exchange here the result :
So my tests were more successful. Here is what I found:
Works fine with a disk read in Shugart mode.
Writing data to the same disk you booted from works fine now.
I was also able to copy a disk to another image. The trick is that you can't use the left/right HxC buttons to select a new image when prompted for a destination by the MR. This always seems to fail. Instead you have to press the middle button, then select the destination image and then press middle button again to select. Then it recognizes the new image and writes to it. It took four passes to copy an entire disk, and the resulting image was an exact copy of the original. **Sometimes** it doesn't recognize a new disk image after being prompted to insert destination. However, this can be overcome by pressing the middle button twice again and having the host retry. Maybe that has something to do with how the disk change line is monitored.
The two images you sent this morning behaved could both be written to.
This failed every time, and the error message told me that the disk I was trying to format was not formatted?!?. Kind of strange, but formatting is unnecessary since you can just copy images and rename them.
So it does work and I think that you can add the Roland MC-300 (and the electrically identical MC-500) to your list of supported devices.