AutoBackup One External To Another

Is there an application that anyone has used that will allow me to backup one external drive (Firewire or USB) to another external drive (Firewire or USB). Basically what I want is the ability to set up a sort of mirrored drive. They don't need to write to one another constantly, I just want to run a full backup on one every night that will make a duplicate on the other.

Thanks!
Mark