Backup from NAS to external drive

Hi,

I have a Western Digital My Book World Edition as a NAS. I also got an external drive (a Western Digital Elements Desktop) to which I want to backup everything from the NAS, and then store it at a different location.

My problem is that I can't find any software that does this, everything I find seems to work the other way around (using the NAS as a backup).

It would be sweet to have the backup working as automatically and simple as possible, so when I wan't to backup my files the next time, it should only copy the files that have changed or are new.

Is there a solution to this problem?

I'm using a Macbook Pro with Snow Leopard.