The SD-35 is actually a sound module and midi file player built into one unit. It plays midi files on a 3.5" floppy disk. I have a unit where the player part of it is bad, however, it can still be used for the sound module portion. So far so good, I haven't gotten the buffer full problem. I have only the MIDI In connected with the USB interface. The SD-50 just came out last year and it's the replacement for the SD-35.. Why it took Roland so many years to come out with a replacement is beyond me. It looks to be a pretty nice unit though!