DVD Burning Problem!

[mac919]

Whenever I try to burn a DVD,I get a "please insert a blank disc" message. This happens no matter if I use Nero(6.6.0.1.5) or DVD Santa. It does'nt matter if I try to use the system burner which is a HP 840b DVD "Lightscribe DVD SuperMultiDrive" or the external Sony DW-Q28A burner.My PC is a HP m7250n Media Center,2.8GHz,750GB Seagate HDD,4GB DDR-2 RAM,ATI Radeon X300se graphics.I should also mention that I have another HP Media Center PC,the model m7360n which has the exact specs and hardware of the m7250n except it has Wi-Fi, an nVidia GeForce 6200SE graphics and the external burner on this PC is a Sony DW-Q30A.I have no problems at all burning on that PC using the exact same software.Can anyone tell me what is the problem?!?!? Please...HELP!!!--Thanks,"Mac"