I tried to follow the guide above using a 2013 MacBook Air and a VMware version of Windows 10 but for some reason, the USB drive (an old SSD from an rMBP in an OWC USB-3 enclosure) would not be recognized by the virtual Windows. I finally ended up using an actual PC running Windows 10 and it recognized the drive without problem. I also had the problem of having an install.esd file instead of an install.wim file. I tried the program suggested in the intowindows link but I guess it's no longer freeware. The comments in that same intowindows article mention a different program, ESD2WIM-WIM2ESD, and that one was free and worked. Just be sure to check your drive letters when following the above guide because the process of converting the install.esd to install.wim will add and remove some virtual drives on its own. I finished creating the drive on the Windows PC and then moved it to my 2014 rMBP and it option-booted just fine. Once I had Windows 8 running off of the USB drive I rebooted to OS X and ran Bootcamp Assistant to get the Bootcamp drivers. Returned to Windows 8 to install the drivers and then did an in-place upgrade to Windows 10 using the save nothing option. That was the longest part of the whole process -- probably took almost 2-hours. Windows 10 finally booted and I again installed the Bootcamp drivers. Whole thing probably took around 3-hours.