I am posting this because I've now had to get Windows 8.1 reinstalled on my nMP (late 2013 cylinder Mac Pro) several times, and each time I struggled for a couple days before I got it sorted out. So I wanted to share what I've learned, and heck, maybe I'll forget one day and have to find my own post! So here's the scenario--I have a Mac Pro with 256GB SSD. I am NOT, in NO WAY, going to shave off 100-200GB of my super fast internal Apple SSD in order to run Windows games. So initially I looked into attempting to use the G-Tech external 2TB USB 3.0 drive I have attached to the system. And truth be told, it's a very bad idea. Windows, as an OS, only wants to be installed in an internal HD. Windows simply does not want to run through USB. That's not to say it's impossible, i'm just saying Windows hates it. So that's when I found out that if you use a Thunderbolt external drive, Windows will see the disk as internal, and a lot of your problems go away. So I ran out and bought a 1TB Buffalo (http://www.buffalotech.com/products/portable-hard-drives/ministation/ministation-thunderbolt) drive for about $150. This made my life a LOT easier. The next hurdle is Bootcamp itself. It only wants to install to your internal drive. So you have to pull some shenanigans to get Windows installed to the external Thunderbolt drive. Namely, you have to force the bootcamp process to stop, and then start installing windows manually at one point. It's not that bad, I'll explain the process in a moment. Here's what you'll need: - Late 2013 nMP (duh) - Buffalo ministation HD 1TB (500GB should work too, but they're nearly the same price) - 8GB Thumb drive (I used a Kingston DT101 G2, which is an officially supported Windows 8.1 install device) - Windows 8.1 Pro x64 .ISO disk image (I strongly recommend getting one of these, and not attempting to use a DVD, because of USB issues that I'll explain later) - Access to a real PC for a moment (to clean the Thunderbolt drive) Directions, from memory, but I should be spot on... 1) Run Bootcamp Assistant from your Utilities folder on your Mac. I used the one included with OS X 10.10.1 Yosemite. 2) Set it up to create a USB install disk. You will be using your 8GB thumb drive. 3) Select the 8GB drive, and let bootcamp create a windows install from your Windows 8.1 Pro x64 .ISO file. 4) When Bootcamp asks how your want to repartition your internal SSD, quit bootcamp. You won't use Bootcamp Assistant any more for this. 5) Now, you need to make sure that the Buffalo ministration drive is cleaned on a real PC. I used a computer my son has for gaming, it's a home-built AMD with win 8.1. In my case, since his gaming rig has no Thunderbolt connections, I hooked the drive up via that USB 3.0 connector that the Buffalo drive has. Then I ran the DOS command "diskpart." I don't remember the exact commands I used (sorry) but the goal is simple, obliterate all the partitions, and even then MBR (master boot record). I think I issued a "drive list" of some sort, then a "drive select", and then a "drive clean." But sorry, my memory is foggy, I used google searches to make this happen at the time. Search "how to clear a drive with diskpart" (I saw this post, which might help) 6) With the newly cleared Buffalo drive connected to your Mac now via Thunderbolt, make sure there are NO, absolutely NO extra USB devices attached. The windows install fails to work if there are any other USB drives. I figured this out because I left my 2TB USB 3.0 mac drive connected, and the windows install kept looking like it was going to work, and then failing in weird ways. 7) Now with the drive attached, it's time to boot off the USB stick that you installed Windows 8.1 on. Well, you created a Windows 8.1 installer USB thumb drive, I should say. To do this, you reboot the Mac, and hold the option key on your keyboard after you hear the reboot chime. Then you select the USB icon that indicates the Windows 8.1 install thumb drive. 8) you will see a Windows logo as the computer boots into the windows 8.1 installer. Once booted, select the language, and enter your serial number. Then you will tell Windows which drive you want to install to. If you took my advice you should see only two possible devices, disk 0 and disk 1. Disk 0, in my case, was the 1TB buffalo as evidenced by it being 931GB unallocated space. Make sure you have the right drive! DO NOT INSTALL WINDOWS ON YOUR MAC SSD. It will erase and obliterate everything. IN my case it was easy to tell which was which by the drive size. Note: You can only install Windows to a drive that has no partitions. You took the drive to a real Windows PC to prep it this way. You used DISKPART to clear everything off. If you don't have a real PC available it *might* be possible to delete partitions that are existing on the drive from this Windows 8.1 installer at this point, but I'm not sure. The problem is that the Windows 8.1 installer sucks so much ass, that it will act like everything is working, and then fail to install and not tell you why. This is what happened when my other 2TB USB drive was still attached. It kept trying to install, and would hang on "copying setup files" then give an error message that didn't really explain anything. If all goes well, it will start installing on the Buffalo, and it might take a really long time. I know that when the installer screen changed to something like "Just a moment while we set up windows for you" I thought the system had hung, because it sat there on that screen for 30-45 minutes. It did in-fact install, but it might have taken 2 hours total. (2 hours? Go Windows!, heh.) Other things you might want to know, are, because you installed Windows 8.1 manually without the assistance of Bootcamp, the Windows thunderbolt drive will not be blessed. That means you cannot use Startup Disk inside the system preferences to set the Windows drive as a boot point. It's an easy work around though, just hold "option" key while booting and pick the Windows drive. I actually opted to install ReFind to make booting Windows or Mac much easier. You can find ReFind here: http://www.rodsbooks.com/refind/ Hope this helps! If you find any problems or want to post any comments, I'll be glad to help. I might amend this post too, if I find things are confusing for folks. Good luck!