Napalmnthemorni- man I wish I could pop my NERO 6 opena nd help ya out - but I am still on NERO 5 and will probably stay there - that said - there has got to be some correlation between 5 & 6.
What you need to burn is called a multisession disk and within in NERO Help there should be a writeup that tells you how to make a multisession disk (with unrelatd files).
Now when I first open my NERO 5 - I get the multiseesion screen and have the option to select new, continue multisession or close that screen. If I pop a previously burned multisession disk into the drive NERO, select continue, and new (I think), the drive will spin up and eventually a screen will come up that displays what is on the cd and I click OK (to select to add to that disk) and it comes up with a burn ISO showing what is on the disk and than it's juts a drag and drop to add to it!
NERO 6 I can't imagine being that different - but you have got to select multisession before burning anything at all to a new cd (so that the cd does not get finalized - as previously stated) and also select "contiune multisession" - and insert the disk and wait for it to spin up and be read!
I hope this is of some help to you - and I hope I didn't skip anything as I am doing it from memory. I know it works fine in NERO 5 as this is how I do straight backups of my files (which I add new files to) and also the incremental backups of things like my address book, emails.