So seperate computer in the shed for the cnc machine. Strip it of all useless software that it doesnt need. All you really need is a basic windows install, ie for a bit of surfing while in the shed, and your cnc control software and possibly another copy of your cad/cam software for some editing at the machine.
Any decent memory stick should do the job. I like Kingston usb sticks, a bit more expensive than some but they work well.
The cnc files are not usually very large so a standard 1GB stick will easily be large enough. Cabled network would be the way to go though.