I agree with Liggy!(Big shock there, I always seem to agree with Liggy...) Double check to make sure it's not a 6100. Dell ships 6100s all the time, which is a 8x DVD+R drive with no support of DVD-R. Dell had been big on shipping +R only drives for a while, and just recently started shipping +/- R drives. If the sticker truly says 6500, it's still possible dell put the 6100 firmware on there.
I would try the suggested TDB firmware and see if you have -R capabilities then. Be sure to save a copy of your current firmware before flashing.