Network Attached Storage security on LAN?

I have a Network Attached Storage (NAS) device on home LAN behind a Linksys
router (w/firewall) connected full time to WWW. The NAS is a backup server
that controls a USB harddrive and backs-up 4 PCs. The NAS is Linksys NSLU2
controlling a 250mb drive. I suppose this is the same issue with the
Buffalo units and any device that acts as a server on the LAN. The NSLU2 is
a Linux OS which I know nothing of.

How do I protect the disk contects on the NAS from being seen or tampered
with from someone on the WWW? Or is the firewall all I need? I not worried
about anything on the LAN, just access via WWW.

Any opinions are welcome to understand my security risks here.

Thank you!

Usually with the router, you can set it up so no internet traffic can
access the NAS. Take a look at the different settings in the router
menu, if that doesn't tell you enough, get a hold of Linksys and
they'll tell you what to do.

Just so you know, yes, it is possible to make it so it's only access
through your local network. of course, if someone takes control of one
of the machines on the network, they might still be able to access it.