Sure is a mystery to me. What are you guys doing??
As I said, I've also had problems with the occasional disc not reading, or reading very slowly then grinding to a halt, but it has always proved to be caused by an incompatible (or faulty, whatever) drive. Those early Kodak CDs, from which it used to take a full 2 minutes on my first computer for an 18mb image to be read and displayed, can now be read at lightning speed on my current XP64 desktop, about 4 or 5 seconds as I recall.
Fortunately, I've always been in the habit of creating space on my hard drives as they fill up with images, by transferring the data to CD and later DVD.