I have two M20s in storage. They both came with the smaller memory on the mother board but I converted the mother board to use 64K chips, on one of them.
I also connected a hard drive to one. The original Olivetti drive is impossible to find. I used a ST251 as I recall ad wasted most of the space. I used a controller
from a TRS80 5Meg hard drive. It did need a little modification as I recall. It needed bus buffers between the M20 and the controller and there was some
modification, I forget what, where the heads cable is connected.
I used a used drive but found I needed to clear Track 0. As the Olivetti expected bad track data there ( I never did find the format of that data but what the PC
put there didn't work. I had to write software to erase the track. Also, you need the right boot floppy version that supports the hard drive. The disk image does
need to be modified. You need to find the two locations that set the track step rate. The ST251 can't step at the rate of the Olivetti drive. You need to change the
two locations to auto-stepping for the ST251. There is a separate location for the read and for the write code,as I recall. It wasn't hard to find as you just look
for the write I/O instructions.
That is about all I recall. I don't know if I wrote this info down someplace.
Both machines have flaky keyboards. I've taken them apart but can't see any reason they should be flaky.
Dwight