I recently bought a 5 1/4 version of Witness from infocom. I bought it for the contents as the listing said the disk was bad. Visually looking at the disk it looked dirty. I used a sharp hobby knife to open the disk "envelope" at the top edge (the edge you hold when you put the disk in the drive). This allowed me to totally remove the magnetic disc itself - being careful not to touch the magnetic parts. I used 90% rubbing alcohol and a cotton ball. I cleaned the bottom side of this disk because that was the side actually read by the head. I used moderate pressure and cleaned the disc in a circular pattern from outside to inside. I could see some dirt on the cotton ball. I let it dry completely and then put it back in the "envelope" and tried it in the drive. Initially it did not work but after repeating the same process two more times the disk has been working. I have used this same process with other "dirty" disks and been successful maybe 30% of the time. David K