Drive no longer works with my media

My Sony DRX-500UL will no longer work with the Ritek 4x DVD-R’s that I use. It has used these discs for over 2 years without a problem but now it doesn’t want to know. I can use other discs fine and the Ritek discs I have work OK in other drives but the two together just won’t work. When I put on of them in the drive it just says that there is no disc. I wouldn’t mind so much except I have over 350 of these discs and I have found in the past that they are very reliable and work in lots of DVD players.

I have flashed with the latest firmware and have all the latest drivers and service packs (Windows XP) but still no luck. Any suggestions?