Last weekend, after searching high and low, I was able to do a clean install of Windows 10 on my Late 2013 Mac Pro (OS X 10.10.4). My primary issue was that in Boot Camp Assistant (v5.1.4), the option to "Install or remove Windows 7 or later version" was (and still is) greyed out. I, like LordVoss, had used Disk Utility to create a Windows partition formatted as FAT.
Boot Camp Assistant had successfully created the Windows install USB key, and I was able to boot into the Windows installer. It was when I tried to format the Windows partition to NTFS did I run into the MBR/GPT issue. I found and installed a OS X command line utility 'gdisk' which changed the hybrid MBR data to allow Windows to install properly. All details are noted here: http://superuser.com/questions/508026/windows-detects-gpt-disk-as-mbr-in-efi-boo t
After using 'gdisk' and rebooting back in the Windows install, the drive formatted without a hitch and I was up and running Windows 10 in no time. I would like to note, and I doubt this has any bearing on my successful install, but I did install rEFInd before I found and installed 'gdisk'. I mention this because I upgraded the bootcamp drivers in Windows last night – to the newly available Boot Camp 6 drivers – and I wasn't able to boot back into OS X. I did see some articles about rEFInd and slow boot times, and I suspect rEFInd as the culprit, but that's a completely different discussion on its own.
My main concern, Loner T, is what option do I have besides creating a partition manually in Disk Utility, if Boot Camp won't create the Windows partition for me?