itunes to ext hard drive transfer

I have an external hard drive that I have music from itunes backed up on. I also have my computer's hard drive that I add music to through itunes. How do I go about syncing up the new music I have in itunes with the existing music on my external drive?

via iLounge Forums http://forums.ilounge.com/itunes-related-mac-pc-applications/278186-itunes-ext-hard-drive-transfer.html