NAS Backup

I've got 2 Dell XPS9000 on a wired home network.  I use a Western Digital NAS to back up the PC's.  I want to move some important files to the NAS so they'll be available to both PC's as well as my laptop - however I need to have the capability to backup the files on the NAS.  Does anyone know of any backup software, suitable for a small home network, that can use the NAS as the backup source and perhaps a external HD as the backup destination?

Thanks for any help.

Jim