Incremental backup from an external iTunes library

I have an external HD attached to my Time Capsule I use as my iTunes library from my MBP and iMac. I'd like to do an initial back up and then scheduled incremental back ups. I have searched but cannot find a program that will allow me to do this. I don't want to copy the files to another external HD as that would be too cumbersome. Any recommendations or suggestion is appreciated.