I’m confused

Nero Burning ROM 12: burns one DVD-R, then won't recognize subsequent blank discs

This problem started only recently. I upgraded to a new Win7 machine, and reinstalled Nero 12. I burned a number of DVDs w/o any issues. Now what happens is that I can burn one disc successfully, and no more unless I restart my machine. I don't understand why this is suddenly happening. Does anyone have any insight into this problem?

Then after this happens, Nero 12 doesn't recognize that I have a DVD drive in my computer. My only option is to burn to "Image Recorder." When I reboot my machine, it recognizes the DVD drive again and lets me burn one disc only.

Thank you in advance for your help.
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