(Posted August 2014, Edited February 2015, September 2018, and October 2018)
You want to run Windows (I used Windows 8 when I did this, but Windows 10 should work too) on your Mac for various reasons, but you don't want to virtualize or take up precious hard drive space with Boot Camp.
Details about the solution provided
We will be utilizing the drivers provided by Boot Camp because they make everything work better (keyboard function keys for example), but not actually using Boot Camp because it is only designed for installing windows to internal drives.
I spent some time thinking about this before I put several peoples ideas together from various tech workarounds to make this possible. This isn’t documented anywhere online that I could find, so I decided to put it out there for others to use.
You will need
a Mac (of course)
access to a PC or virtual machine running Windows for the use of WinToUSB and the Command Prompt (as of the writing of this How–To, this is the easiest and fastest way to do this.)
a copy of Windows in ISO format
a USB thumb drive
a USB 3.0/3.1 drive (recommended over 2.0 for speed, and can have other I/O options like Thunderbolt, but USB is necessary for the WinToUSB software).
For best results, use Boot Camp utility to create your thumb drive with Mac drivers. This is all you will use Boot Camp to do.
For the Boot Camp options, leave the box checked that says “Download the latest Windows support software from Apple.” Uncheck “Install Windows 7 or later version.”
As of right now, version 4.1 doesn’t seem to work at all on a virtual machine, at least not in my experience trying it with Windows 10. Grab an older version from here: https://wintousb.en.uptodown.com/windows/old if you are getting the error “Failed to extract file from WIM file.” Version 4.1 works with a real PC, so maybe it is something to do with the virtualized environment.
Option 1: Go to the Windows PC or virtual machine running Windows and launch the Command Prompt (go to Run and type in CMD and hit enter).
Option 2: If you created a bootable USB install drive with Boot Camp and want to use that instead of the Windows PC or virtual machine, boot the Boot Camp created Windows install USB drive by holding Option at reboot of your Mac and selecting the USB drive (called EFI Boot). At the startup screen, run the command prompt by hitting Shift+F10.
The objective here is to setup the USB 3 drive that you intend to install Windows on so that it will have an EFI partition, the Windows required reserved partition, and free disk space for the install.
Type the following commands, excluding the ‘ ‘ around the commands (partially acquired from here):
‘diskpart’ (this puts you into the windows partitioning shell)
‘list disk’ to list all the drives available. You want to make sure you don’t select your Mac’s main HDD or the USB thumb drive for the next step. (We will call the correct drive ‘Disk 1′ for keeping the rest of the instructions clearer.)
‘select Disk 1′ (you will see the command prompt say that disk is now the selected disk)
‘convert gpt’ (this converts your hard drive from an MBR partition table to a GUID partition table)
‘create partition efi size=200′ (this creates the efi partition where the bootloader will live)
‘format fs=fat32′ (this formats the EFI partition as fat32 so that Windows can write to it)
‘create partition msr size=128′ (this creates a Windows required reserved partition)
‘create partition primary’ (this uses the rest of your free space to create a usable partition)
‘format fs=ntfs quick label=Windows’ (this formats the Windows partition and labels it as “Windows” (or whatever you want it to say) which is what OS X will see)
‘exit’ (this exits the windows partitioning shell)
Connect the USB 3.0 drive to the PC with WinToUSB installed on it.
Select the ISO with your Windows of choice
Select the Partitions (EFI etc. will only be available to the places that were formatted for them )
Boot that drive and you will finish the Windows install. Then run the Boot Camp driver setup and you are all done.
Windows seems to run a little hot on my Macbook Pro. I think it doesn’t turn the fans up until it is really hot. I use Macs Fan Control (Make sure to get the Windows version.) I like Macs Fan Control for OSX, also. Keeps things cooler. I set the range pretty wide so that the fans come on earlier and quieter. They will rev up if they need to when the temperature reaches the top temperature setting that you choose.
For brightness issues, check out this link: http://www.howtogeek.com/107173/disable-windows-8s-adaptive-brightness-to-fix-dark-screen-problems/
One last note before you go. DO NOT USE DISK UTILITY TO REPAIR THIS DRIVE. It will mess up the partitioning you did before you installed Windows to the drive.