CD / DVD burning verification was mentioned earlier, but maybe it would be the best to put a checksum on the disk during the burn. So all data on the disk could be verified later on to check if it is useable or not. I allways put such an MD5 verification file onto my disks since an important one had gone wrong. Of course it doesn't help much, but at least you may know what you lost and maybe it helps to find another backup and copy it in time.