sounds like it's not the dvd drive then, but your burning software, it seems it's not recognizing the dvd drive.
the old versions of Roxio easy cd creator would just all of a sudden stop recognizing the drive.
it happened to my girlfriend & i went to the Roxio site & downloaded the update, and that fixxed the prob.
So i would go to the Nero site & search for an update for the software, they may have a fix for this.
if all else fails i would just uninstall Nero & re-install it.