Ok, little update, the MFD-51W-03 is WORKING! Tried it this afternoon when I got offline, and it seems to be fine...I can read, write and format without any problems.

Had a bit of a play with the SuperDrive as well...I pulled out the head assembly and had a bit of a look, I moved the spring to one of the lower steps, to bring the top head closer, but it still didn't help much. I took the spring out to have a look, and the spring itself is fine. I wonder what the go is...oh well, at least I've got one drive working.