A Mac app for harmonizing data in two folders?

I'm transferring Media files back and forth between a number of drives. Is there a Mac app that will sync up two folders on separate drives ... match the folders and their contents? Or compare the two folders so I can quickly see what is missing in one?

I'm doing a Final Cut Pro X consolidation ... moving a Library from one drive to another. The FCPX consolidation moved a whole bunch but not ALL of the files.

I want to bypass FCPX and just move-by-hand all the missing files/folders from the old drive to new drive ... without having to visually analyze back and forth between the two to get the right folders/files moved.

I know I could just drag and drop and have the pop-up tell me I'm about to copy-over or double-up my files... and hit the right choice to not create doubles.

But I'm wondering if there is an app that will be a bit less unnerving ... and make it easier to sync up the drives?

All ears,

Ben