I don’t know of any NAS devices that will allow both a USB and network connection at the same time. What is your concern with having the warehouse computer on the network? You can use a variety of methods to limit the warehouse PC’s access to network resources as well as Internet. One simple way to eliminate Internet access is to leave the gateway setting in the network TCP/IP panel blank. This will allow the PC access to resources on the local network (which can be controlled by file sharing permissions) while not allowing access to the Internet. With a little more work a setup using SmoothWall could allow you a pretty good level of control over what access various com0puters have on the network.
I think the problem here is that you are confusing network types!
USB is usually a master/slave type network, whereas TCP/IP (What NAS does) is a peer-2-peer network.
So USB is really a peripheral bus, rather than a network connection.
But as “Flame” has already said, its quite possible to setup a NAS box that has a link from your Factory machine (maybe a direct link via a crossed cable) to the NAS box that is not routed to the Internet and a separate link (ie another NIC) that is connected to your IntrAnet that alloows you access to the same data.