OS X :: Automatically Copy Certain Files?

I want to copy certain files (to an external hard drive, but that doesn't matter) to another folder. But i want it to do it automatically with one click (or two, but you get the point). I want it to only copy files that are also in the destination folder from the source folder to the destination folder. (sorry that doesn't make much sense). But my goal is to back up my music into a folder, but not all of, only the folders that are already there. So basically, replacing old with new, but adding no new folders at all.