Sorry for interrupting this conversation, but...
By the way, avoid Ritek's 8x DVD-R (dash) media, RITEK G05.
It's known to have bad stability.
(Ritek's DVD+R (all speeds) and 16x DVD-R are ok though.)
Check your discs for readability!
If transfer rate tests (CD-DVD Speed benchmarks) don't look good anymore, it's time for a new backup.