Backup NAS to an external drive on USB port

Hi there,
Has anyone in this forum found a suitable method to backing up their NAS to an external USB drive. (Poor man's raid I suppose).
I used to back up my machines to local USB hard drives and then run windows based software to back them all up to the NAS. Now I've got a Mac in the mix and am thinking it would be more efficient to just back up everyone to the NAS and have a backup of that as well.
It isn't immediately obvious to me how to do this, so if someone is already doing something like this and willing to share a recipe I'd appreciate it!
Cheers!
Has anyone in this forum found a suitable method to backing up their NAS to an external USB drive. (Poor man's raid I suppose).
I used to back up my machines to local USB hard drives and then run windows based software to back them all up to the NAS. Now I've got a Mac in the mix and am thinking it would be more efficient to just back up everyone to the NAS and have a backup of that as well.
It isn't immediately obvious to me how to do this, so if someone is already doing something like this and willing to share a recipe I'd appreciate it!
Cheers!