Welcome to the community, RJ 45ACP!
I'd suggest you to unplug all other storage devices except the one you are planning to install Windows 8.1 on. This failure might be happening because of all the drives connected to the motherboard at the same time of the OS installation, which is most probably causing the common Windows confusion that appears when there is more than one storage drive that it can install on.
Give it a try and let me know how it goes!
SuperSoph_WD