Indeed, in my experience, there's some kind of biofilm that forms on the disk surface. Doesn't look like anything more than a thin stain or discoloration, but disks so afflicted never read correctly. Suspect that whatever it is, it's between the mylar and the oxide layer so the bits literally flake off when the head hits those patches.
Why do I think it's biological -- just the shapes of the patches resemble what you see in laboratory petri dishes, and that the amount of such corruption seems to correlate to humidity. Disks from an attic seldom have this issue, compared to ones from basements.
Why do I think it's biological -- just the shapes of the patches resemble what you see in laboratory petri dishes, and that the amount of such corruption seems to correlate to humidity. Disks from an attic seldom have this issue, compared to ones from basements.