How do I stop the software syncing music already on my Zune?

Basically, Ive moved my music library from my computer onto an external hard drive. I have told the Zune software where to find the files and its updated its library accordingly. I have it set to automatically sync new items to my Zune. When I connect my Zune, it goes to sync the entire library because I've moved it to the new hard drive. Obviously I dont want to do that, but I want to keep it set to sync new stuff. How do I get it to stop trying to sync everything, and keep the automatic function?