One of my dearest old friends from Michigan, David, was in town this past week. He came over for a visit last night. We just talked for hours about synthesizers or whatever, playing with things here and there (I was showing him how great it is to sequence with a TR-808 and CSQ in unison). Anyway, I have an old Roland MC-500 Micro Composer that I got a while back but never quite got working. David had one years ago and knew all about it. Apparently there's a lot that happens with those floppy disks. There are two different types of floppy disks: the older kind, DD or Double Density and the newer kind, HD or Hight Density. See figure A.
It usually says somewhere on the disk if it is DD or HD, but another way to tell is that the Double Density disks only have one write protection tab and the High Density disks have two (that isn't really clearly displayed in figure A). Anyway, the MC-500 only works will Double Density disks. Where can you find them? I don't know, probably somewhere on the internet, but I'd say the cheapest way to get them is to pick through boxes of old junk at thrift stores.