External drive enclosures are a dime a dozen. I just bought one from micro center not long ago for like $13. But you said "it would just be a server?". Did you mean it would contain the operating system? If so your laptops motherboard would have to support booting from a usb devise and you might have to configure your bios to do so. Like moving your usb port to the top of your boot priority list for example. Also I personally don't know if Windows will run in this (portable) fashion. I'm pretty sure there is a version of Lennox that does.