Looking for a drives comparasion program

I decided it's time to upgrade my drive to a higher capacity. I installed the larger drive and copied over all of the files from the old drive onto the new. The new drive (750GB) says that 460GB is used but the old drive (500GB) says that 485 is used. This leaves 20GB of data difference. I have copied over twice already and there is still a difference of 20GB. Is there a free program out there that compares the data of these two drives and tell me what is missing on the drive that is lesser drive usage?

I have all hidden files displayed. There should be no system files because these are mostly home movies videos and music and pictures. I have found some programs that check for duplicates. But these programs check for duplicates within the same folder or drive. I need a program that do a comparasion between two drives or at the very least two different folders. It's simmliar to the "diff" command in unix.