Problem with LG GSA 5160D

I have the above drive, its an external connected via the USB interface. The firewire doesn’t like being turned off and on for some reason.

Anyway, I’ve burnt a few films onto 4.7GB DVD+RW discs without problems, but I can’t then erase those discs and burn something else onto them.

I’m using the latest Nero software, the discs are Sony DPW120A2 and it does say ‘rewriteable’ dvd+rw on the case so I think they’re ok.

Any ideas? It just will not format them again, comes up with errors. I’m a noob at DVD burning btw.