Ok man it's simple... The reasons Nero won't burn that DVD is because it's already been written to. It sounds like you're trying to copy a DVD right? Well you can't just pop it in there and tell Nero to make a duplicate, so that's most likely your problem. If you have some spare DVD BLANK disks, you might just want to try burning some regular data (maybe photos or something) to the blank. That will help you see if it's working at all.
After you do that, might I suggest a few tools to help with what seems to be the issue you're having now:
DVD Shrink -
http://www.dvdshrink.org/
If you download it from the mrbass.org link it also has a basic guide for using it. This one will rip, compress, and burn (using Nero's API) the DVD in question.
Personally I prefer to rip initially using DVD Decrypter:
http://www.dvddecrypter.com/
I think this little application is incredible. I use it to rip the movie in question and then I use DVD Shrink to do whatever I want with it. Works great.
If I am totally missing the point of what you're trying to do I'm sorry, but from your second post I don't think I am. Good luck.