Copy files that are different only between two large volumes?

I'm moving a large amount of data from an old server to a new server, I'll have a small windows to swap the servers over so I'm copying all the data in advance (300GB of docs) and then in my small windows I want to just sync the files that have changed which may just be a few GB, XCOPY doesn't seem to be able to do this, any suggestions?

Thanks Steven

Either of rsync (Unixy) or Robocopy (a more windows-native application, and it comes with newer versions of Windows, I believe) should do the trick fine here. They compare the source and destination file trees (they send checksums of the files rather than the whole file, for comparison purposes) and only transfer those bits that have changed.

I've got a lengthy writeup on how we did the migration of Github to their new setup; that involved copying several terabytes of filesystem data and other stuff (databases, etc). Hopefully it'll give you a few ideas.