SyncTwoFolders is a simple FREE app that will sync two folders. I've gotten mixed up with a project that's messy, to put it nicely, and while FCP's media manager is great, in this particular case just hooking up the various hard drives and syncing up the folders is what's needed initially before the real media management can take place. And by various hard drives I mean the occasional USB flash drive. Seriously. Messy.
So looking around I found SyncTwoFolders today and it's not bad. Plus, it's free. It wasn't originally written in English so keep that in mind when you find an odd phrase or word choice in the app.