So I want to get my windows 8.1 64 bit pro installed to a drive in my Mac Pro 2009 running El Capitan. I have an iso file and my license key all ready for the install but I run through boot camp and when I reach the step to select the Windows install from location it keeps asking me to insert the disc. I read that I should put the iso file on a USB drive, that doesn't help, I tried mounting the iso file and that didn't help. Apple's guide seems to suggest it works with an Iso file as it says to extract the contents of the install disc to an iso. The only thing left is to burn the iso file to a disc, but that seems redundant. Surely there is a way I can do this? My copy of Windows if the full retail, but came as a download version. Am I going to have to buy another Windows license with a physical disc?!?!?