That's a good reminder of a couple of things. First: it's common that a CD/DVD that won't read in one drive will read in another. Margins/tolerances are incredibly fine. The second is, of course, always to ensure you have backups of backups - especially when it comes to CDs/DVDs. There have always been horror stories about how long they remain viable/readable, and it's wise to ensure that you a) don't have just one copy, and b) refresh/replace them periodically.
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