Do you use DVDs (which ever format) to backup your photo files? Do you use Windows XP? Do you use multisession (so you don't have to burn everything at one short)? If the answer to all 3 is yes then beware!
I only recently discovered this problem with the combination of the 3. I can't read anything other than first session of the DVD if the total size of the data exceeds 4gb. In fact, this bug has been documented and the fix will be out with the Windoze XP service pack 2.
More info about it here.