So I just tried installing Windows 8.1 on my new MBP Retina 15 inch....I have done it before without any issues, but for whatever reason, this time it didn't work. I used Boot Camp Assistant in order to download the necessary files to have Windows work properly on the MBP, as well as partitioned the drive for installation. After this, my laptop restarted and nothing happened, so I restarted again and pressed the alt/option key in order to boot from the windows disc that I had connected via a external USB DVD drive. From here, I go to the partition I had chose for Windows, which was roughly 140GB in size. I try and choose the "Next" option in order to begin the install, but it tells me it cannot complete the action because something about the partition being GPI or something of the sort. I try choosing the format option, and still nothing. Any suggestions? FYI, this is a 4th Generation MBP Retina 15 inch. Thanks guys!