Dude:
I don't have an S-50, but I had an S-550 for a little while, and they are similar devices, so I thought I'd pass this on in the hopes it'll help
.
1) If you're in the US, contact Roland US and request a new disk. I did this for my S-550, and they sent one at no charge... I had to wait for a couple weeks, but at least I got one!
2) Regarding SCSI, both myself with the S-550 and my friend with the S-50 fought long, arduous battles with Roland, other users, and search engines on the Net trying to find out how to incorporate a SCSI bus to offload data. We completely struck out! There was one company about 1985 (who's name I've forgotten) who made a SCSI interface "kludge" for the S-50 - it came as a kit which required about 18 hours to install, and the high point was an IC which needed to be piggyback-soldered onto the CPU addressing chip. My friend bought it (it cost about $500 at the time) and installed it, and damn near destroyed the sampler in the process - the display has not worked since, and it does other random weird stuff. We never DID get a hard drive to work with it.
Given that an ESI sampler can be had used for $495 nowdays, my honest advice is to give the idea up and save yourself the stress: you're better off spending your time making music.
Rock on. Teejay