Nero has an option to select a burning speed.  If it fails at 10X, dumb it down to 8X and so forth to see what the threshold is.  This will give you an idea if the drive is toast.  If it does burn at a lower speed then issue solved.  If not, there's proof the drive passed away quietly.

Not neccassarily.  Try a different burning app than Nero and see if the same results apply.  I had a similar issue with Nero that led me to use Roxio in the end.  It may just be an issue with your burning app.  Try that first.