I find Gyula's navigator a good tool for that. Worth a try, because it's free. Download from http://download.cnet.com/Gyula-s-Windows-Navigator/3000-2239_4-27561.html
Or use xxcopy (free download also, but you need to feel a little bit comfortable with the command line) with backup switch to backup everything you forgot.
Or use Syncback or Microsofts synctool (free) to to the same via the GUI.
I'm backing up two computers to the same external HD. I have tried to keep the same files in two large folders across the two machines, but wanted to make sure I have everything without duplicates. Is there a good program that looks at two large folders and tells me what I might be missing in one?