Any disk drive
that can give you a command prompt of sorts is still
pretty cool though.
The oldest one I know of is the RA81, and what's more, I've _used_ the
serial port to dignose HDA problems. Very handy.
The R80 (and presumaly the RM80, I don't know aout the RA80) had a little
'front panel' inside on the processor PCB. A couple of thumbwheel
switches, a couple of toggle switches, and IIRC 8 LEDs. You could do some
interesting diagrnositcs with those, IIRC you oculd even read out any
location of the control processor's (8085 I think) address space.
-tony