Here's another thing I'm worried about, though it does give me an explanation for all this. I looked at all the discs that were giving me problems, and at closer inspection, the discs are pretty badly scratched. So, at least I can be fairly certain that this is what's causing the problem, and that the other discs that aren't scratched are okay. But what I can't figure out is when the discs were scratched. Could I burn to a scratched disc? I just don't know what I would have done to scratch it, particularly since the scratches are in a circular pattern around the outer edge. They're pretty severe: