Problems installing Windows 8.1 using Boot Camp...

I have a 2008 mac pro (intel) with two internal drives running os X 10.10. I have ISO files for both windows 7 and windows 8.1. I've used bootcamp to make various attempts at bootable USBs and have properly partitioned my second internal drive for windows. However - whenever my mac attempts to boot from the USB drive it goes to a black screen with a flashing cursor and nothing else happens.

I would attempt to burn a DVD from the ISO files but my superdrive is broken. When I hold down alt on startup the USB does not show. I've installed windows on this machine before without problems but I have always used the DVD drive.

I don't want to buy another superdrive as I would only use it for installing windows and nothing else. Can anyone tell me if there is another way to get this done?