I too, am experiencing a similar if not identical problem. I have been using Sony DVD-RW discs almost ever since I bought the unit about one year ago. The other night, I was copying a movie from the HDD to the disc and the power went out for about 5 seconds. Damn! When I brought the unit back on line and it executed scan disc, I was able to continue with the copying (after cleaning up that disc). I then copied another disc just fine that night and a few others a couple days later.
A few days after that, I began picking discs out of a different carousel of 25 blank Sony DVD-RWs and got one that wouldn't perform the quick prep --- it went through the initialization process and then slid open the tray with no error message. I closed the tray with same disc and it said "INVALID DISC". So I picked out another disc and it prepared just fine, so I just tossed the first disc. It progressively became worse very quickly and after about five discs in the stack of 25, all discs began showing invalid!
I tried an audio CD disc and it went fine, then tried erasing an older movie off a DVD-RW and it re-copied fine, so I kinda thought it might be the discs themselves. Bought another carousel of 25 Sony DVD-RWs and every disc gave me the same result.
I then tried Memorex DVD-R and got the same response.
It will playback all movies and all types of discs (burnt or pre-recorded) just fine and it will record to a disc that has been previously prepared or initialized.
After reading the posts here and elsewhere, I am thinking that the lens is simply dirty. My reasoning is that it progressively got worse, but did not fail immediately at the time of the power failure.
I am going to try a cleaner and see if that cures it before I move on to more elaborate options.