Software to ID duplicate and unique files?

Help. I have two large capacity Hard drives. Ive copied files between these drives and have also used auto-info-transfer to update info and files on these drives a new iMac. So I am now in a position where I am uncertain which files exist on both drives and which files are unique to either drive. I want to re-format on of the drives without loosing any unique files on that drive.

Can anyone recommend file management software that would insure that I would not mistakenly delete any files that I have not already copied to another drive? Ideally, I am looking for software that would allow me to find files that are on both disks (duplicates) and files that are unique to a single drive.

I won't bore you with how I got myself into this situation and I know back-up software, if it had bee in place would have helped me manage this all along.

Thanks