A NAS drive like I have can be accessed anywhere. You get you ISP to put you on a static IP address. Might pay a couple of dollars extra but if you really need to do this it's minor. Next when you're on a static IP you should have 5 IP addresses that come with the extra money you're paying.
Get one out of the list activated and get the gateway IP, subnet mask and the dns server IP's. Program those in to the NAS.
Now the fun part.
You have to have a switch connected to the modem. Connected to the switch will be the NAS and the router you're using. The router has one IP and the NAS has another. Both can get online at any time in that configuration.
To access the NAS you can either map the drive on your laptop or ftp in. I go either way on mine. Just don't hand out your NAS address and you're relatively safe.
I have a NAS-100 I got from a company here local. They don't ship but here's the addy just the same. http://www.intrex.com/parts/parts.aspx
Look in drive enclosures under 3.5" drives. It's called Speed Dragon.
But whatever route you go, shop carefully. There's some real crap out there.