Sync 2 external directory trees?

I have my Itunes folder on an external drive, and a few weeks ago upgraded to a bigger disk.

Copying the files from the old to the new disk was a bit of a pain, as the copy process fell over a couple of times, and I ended up finishing it up manually - or so I thought.

Anyway, it has become clear that I have some files missing on the new disk, so I want to copy them across.

The problem is, being an Itunes directory, this would mean manually comparing several thousand directories - I don't want to recopy, as I already know it won't work, and I now have new stuff in the new Itunes directory that was never on the old one.

So, any idea of a utility that will tell me what is on the original disk that isn't on the new one? Have googled and tried a few utilities, but nothing seems to do anything remotely like this. I'm happy enough to move the files manually, but I need to know what they are first . . .

I did think about having dreamweaver do it, but I'm just a little wary of watching it trash both copies, if I set it up wrong.

Thanks for your help.