I read your post carefully & the point I'm making is that just importing into a project does not mean they are copyied from the DVD that you subsequently remove. Once removed Nero has no way of accessing those files 'cos they aren't there.
So what I'm saying is to copy the files from the DVD with Explorer onto your HDD so that when you import those copied files that are on the HDD into the project it doesn't matter what you do with the DVD that they were on as they're now accessible on the HDD.
But as you say you've done that then that can be ignored I guess.
The reference to DMA in my post is actually in my signature & not related to the points I'm making.