'Diff'ing for existence, not content

I am backing up my My Documents folder before wiping my hard drive. I just dragged it to my external hard drive - it's 90GB and took an eternity to copy.

Before I wipe it, I'd really like to confirm that everything made it - the first time it seems like some hidden files or read only files didn't copy.

Is there an application or technique I can use to diff the two copies of My Documents for file existence? i.e. It will report if a file exists on my C drive that is not on my external?