The back up method

On each server, the root account sets up a script that does the following:

Secure the mount point to be owned by root, and set it up to accessible only by root.
Mount the NAS as to a local mount point, using a command like
mount -t cifs \\backupmaster\tarfiles /media/share -ousername=backupgami,password=insanepw,uid=root,rw
Of course, insanepw is an insanely complex password that no one can guess or crack.
Once mounted, the root user backs up the files using tar and all the database using some database dump utility.
At the end of the backup process, the share is unmounted.