I try to consolidate old drives to new ones of larger capacity. Sometimes files have been renamed, but are otherwise identical. Sometimes an old directory has just a few more files in it than a newer directory with the same name. Sometimes a file has the same name but the size differs.

So I often find myself asking the question:

Are there any files on this old drive or directory that I haven't already copied to the new drive?

I just want to know that I have the files, I don't want to try and sync stuff automatically (Syncing tools tend to just sync, creating duplicate folder structures and other problems, so I prefer to do it by hand).

Basically, if an old drive has a file called "foo.bar" ten directories deep, and my new big drive has an identical file called "oldstuff.zip" in the root, I just want a "yes you have it" or "no, unique files exist".

Is there a free tool, a script or a quick and easy method (Mac/Unix or Windows) to get the answer?