Would it help to rewrite the magnetic disks, thus renewing the (ummm, I dunknow) magnetic flux of the original disk? If so would I risk screwing it up because of some copy protection scheme. I screwed up my Maxiplan master disk trying to change the icons on it to the 2.0 dos colour scheme.
I'm now leery of CD-Rs after seeing that some degrade after as little as 2 years.