Network attached storage security?

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 all running XP SP2.
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!