Script for new file copy only?

hi,
im running osx ML on an intel mac and i'm a newbie to terminal and scripts, what i want to try to do is the following :
on my mac i have my mp3 directory, on an external hard drive i also have my mp3 directory, BUT when i get new cd,s/mp3's i put them onto the mac. i need a script/terminal command so i can copy FROM the mac TO the external drive and instead of spending ages copying all the files it will only copy the new/changed files. I used to do this on my old pc but dont know how on a mac.
any help is greatly appreciated.

 

Thanks guys, CCC does what I want,but I wanted to create a shortcut on my desktop that linked direct to the script rather than opening a program. Like I said tho, CCC does do what I want.
So thanks