Shouldnt putting a WEP on your router help?
That would (slightly) help prevent wireless
access to the share, not access from across the internet.
(BTW: WEP sucks. Very easily broken. Wireless should be run using WPA2-AES whenever possible.)
My question is how can I protect this drive from people accessing it from outside the network (ie. the Internet).
Unless you've specifically forwarded the ports used for file & print sharing to the drive (or, specifically made it available (in the router config) to the internet (and, why would you do that?)), that should be a non-issue.
By its very nature, a router separates internal devices from the 'net - all that you 'see' from the outside is the router itself, plus anything that you've specifically forwarded a given port to. If you've forwarded no ports, you 'see' nothing but the router.
(side note: Best Practices for reducing your attack surface includes disabling 'remote admin acccess' to the router. If you haven't yet done so, you should, unless you have Very Good Reasons to leave it enabled.)
Want/need to (dis)prove that the shares on the network drive are accessable from outside the house? That's easy to do.
First, note your public IP address (you can get this from the router's admin pages.) Next, go off-site - work, library, Starbucks, whatever. Point Windows Explorer to \\<public_IP_address> - do you get any response?