No shit? Not sure if trolling, but try booting Linux from a USB instead so you free up the disk drive.
Why trolling?
Btw if it's the only solution avaiable I'll do it through usb live, thanks.
Well, you ejected the disk that Linux is running from. Would you ask why the music on a CD stopped playing if you took out the CD?
Its It's not a good idea to store wallet.dat into a DVD.
Why's that? It's as good as any other, though I wouldn't just have one back up on 1 DVD. I'd have several on several different mediums - USB etc.