I'm using 2 of
http://www.buffalo-technology.com/
Config through a webinterface, can set access permission per share.
But most professional NAS solutions have that, including LDAP or other directory related security measures. On the other hand, u can create a file server easy (Windows, Mac, Unix/Linux). Using remote control it provides the most flexible way to setup such a file server. Then you can use internal or external drives.
It's best to search for NAS solution on the internet. They vary from a few hundred $ to seriously tens of thousands, multi disc, RAID 5 solutions.