I've just used a MacBook Air to create a Windows 8.1 install USB. Worked great. It just crossed my mind that the drivers from Apple are probably device specific - ie. specific to the MBA - and I might need to rebuild the installer from the 5K iMac I intend to use it on. Not a problem if so, I was trying to make efficiencies for tomorrow. Will a Windows 8.1 USB installer built via BootCamp Assistant on a MacBook Air work on a 5K iMac? Happy Christmas all.