Here is a file handling trick I had to find for myself. It works in OS X, I haven't tried on Windows.
Say you have folders A and B. All the files in folder A are also in folder B, but not vice versa. And you need to isolate the files in folder B which are *not* in folder A. How to do it? Manually is tedious and error-prone.
1: Make sure you have backups of both folders.
2: Select all the files in folder A, and drag them to folder B.
3: When Finder asks you if you want to replace the files, select "apply to all" and click "replace".
4: Use Edit--->undo.
This last bit is the smart part. It will put the files back in folder A, but it will *not* replace the deleted files in folder B! (I think we can call that an error in the OS.) So you now have a folder of what you wanted, all the files which were in folder B but not in A.