Looking for a NAS with these requirements

Hi, I am looking for an external storage device/solution that fits the following requirements
1. It is portable
2. Supports USB, once connected, it becomes an external harddisk.
3. Supports cable network, once connected, it becomes some kind of file server, which I can upload and download files, both within LAN and outside with FTP/HTTP.
4. Support 2 and 3 simultaneously. So it has to support multi streaming.
optional. Better to support Wi-Fi rather than cable, in case there is no network/router, I can use ad-hoc.

Point is, I have 1 desktop not connected to network, it needs this external storage through USB (2.). While connecting to my desktop, this external storage also connects to the network, so that my laptops can access it either through LAN (3.)or Wi-Fi ad-hoc (4). Good thing is I can take it with me to my work (1.).

A friend of mine told me about NAS. I think it is quite close to what I want. Just that I am not sure if NAS can be used with USB and network simultaneously. I checked a few NAS but only network connection is provided.

Thanks everyone