writable DVDs degrade, but a typical DVD won't on its own for a long time in reasonable conditions. I suspect that over the years those library disks have been

writable DVDs degrade, but a typical DVD won't on its own for a long time in reasonable conditions. I suspect that over the years those library disks have been scratched to hell several times over and ave had to be buffed down to the point where it's causing problems even though they now look clean.