App to compare directory file-structure/contents?

Quick question:

Looking for an app that will help me compare directory file-structure/contents. Essentially we have to make backups of our video content here at work and we have it on two seperate external harddrives. The thing is I want to easily be able to make sure that these two drives are mirrors of each other, or rather, that they contain all the same content.

The content has already been copied over and has come from a variety of different machines so doing something like a clone is a bit out of the question at the moment.

Essentially what I'm looking for is an app that would be able to look at the file-structure/contents of each drive and compare them -- basically to say "both drives contain these folders that are this size and their contents are the same but drive A has a folder that drive B doesn't have" and then I know to make sure to bring said folder over.

Is there an app to do this? So far I've been doing it through finder and its slow, and tedious, and already have missed things.

ANY help would be greatly appreciated.