"It was Nero 9 that started the original problem"
It would have been useful to have known that to start with so I could have posted relevant links or information instead of me rattling on about Windows built-in burning utility.
All I have to add is that I steer clear of making multi-session discs as they can cause problems later on, especially when you want to read them on a different PC, and it's not unknown for one or more sessions to disappear even after finalizing. Also, multi-session writes are less reliable than regular, single-session writes.
DVD media is now cheap enough to make multi-session writing unnecessary anyway.