thank you for your answers. This is exactly what I want to know.

Alright, so this thing is called NAS server. Yeah that's what I need. The SmartStor NS4300N with its 4 slots sure is a good solution for the future, also the Drobo. The TS-109 Pro with one drive is probably the cheapest solution for professional use.
I also thought that a network drive can access all OS's, but I made bad experiences with the one I already have. It needs a software installed.
I think the three above work without special software and can be configured with a web browser. That should work on all OS's.
I have not yet found a dealer for the TS-109 Pro in my country, but I guess all web-based network drives should work for my purposes. I intend to put textures and footage there and work with two computers.
Thanks again, guys, I can't wait to have such a NAS server running here. That will be a big help for work. If you have some additional tips (what letters to which drive, IP configuration, etc.), I'd be pleased to hear.