It's absolutely possible to run BootCamp from an External Drive (ie : windows installed on an USB drive)
The windows installation program refuses to install itself to an external USB Drive.
That's not a problem, there's a way to install windows installation files on an external USB drive, and once booted from it, Windows will finish to install itself from said drive.
I tested it with Windows 8.1 x64 and it work perfectly from external USB drive!!
here's how to do so :
You will need :
- your windows 8.1 ISO file (choose x64 because Apple Bootcamp tools are meant for x64)
- a running PC (on windows 7 or 8) real or virtualised
- a blank external drive (at least 16GB)
/!\ WHOLE DISK WILL BE ERASED, I cannot be responsible for data loss /!\
this software :
- Apple Bootcamp drivers (obtained from the BootCamp setup)
- Microsoft AIK tool : http://www.microsoft.com/en-US/download/details.aspx?id=5753
- Power Iso (create a virtual drive, unless you're using a Virtualised Window) : http://www.poweriso.com/
If you don't have Windows ISO, you can download a demo there : http://www.microsoft.com/en-us/evalcenter/evaluate-windows-8-1-enterprise
or try windows 10 : http://preview.windows.com/
On your existing PC Setup, do the following steps (each command will be in uppercase to increase readability, type them in lowercase, if there's a space between words, insert a space obviously) :
1 - install Windows AIK, once done navigate to this folder : C:\program files\Windows AIK\Tools\Amd64\
2 - in this folder, move the file imagex.exe to the root folder of your C:\ drive
3 - launch the CMD line tool (clic on the Start menu, in the search bar type CMD, then launch it from the menu)
4 - at the prompt, type DISKPART and hit Return Key
the command line will be like this DISKPART >
5 - type LIST DISK and hit Return Key
It will list each available drive on your PC with a Disk number (DISK 0 ; DISK 1; DISK # ; etc...)
6 - Choose the disk you want your windows 8.1 installed onto and type accordingly SELECT DISK # and hit Return Key
(# being the number of the disk you want to use (ie SELECT DISK 2) and hit Return Key (according to our example the prompt will state that disk 2 is now the selected disk). Just to be sure of what you did, you can type LIST DISK again, and you will see a * in front of the disk you will erase and prepare for the installation of windows. If you want to be absolutely sure that you selected the right disk, you can unplug the disk and type LIST DISK again and repeat the above steps.
7 - Type CLEAN and hit Return Key
DiskPart succeeded in cleaning the disk
8 - Type CREATE PARTITION 1 and hit Return Key
DiskPart succeeded in creating the specified partition
9 - Type SELECT PARTITION 1 and hit Return Key
Partition 1 is now the selected partition
10 - Type ACTIVE and hit Return Key
DiskPart marked the current partition as active
11 - Type FORMAT FS=NTFS QUICK and hit Return Key
DiskPart successfully formatted the volume
12 - Type ASSIGN and hit Return Key
DiskPart successfully assigned the drive letter or mount point
13 -Type EXIT and hit Return Key or close the window
Following 14 to 16 steps will allow you to have the window 8 ISO mounted as a drive. If you are doing this with Parallels Desktop or VMWare fusion, just mount the .iso file as a drive in the CD/DVD Drive section of the software and go to step 17
14 - Install PowerISO
15 - Clic on the "Mount button", and select "Set Number of Drives Menu", choose "1 Drive"
16 - Then clic on the "Mount button" again, then "Mount Image". Select your windows 8.1 ISO file and clic "Open"
Following steps are for everyone
Please note your drives letters before proceeding :
What is your Windows CD drive Letter ? My Windows Drive letter is E:
What is your external Hard drive letter ? My USB Drive letter is G:
So remember to change the letters for your system accordingly.
17 - Verify that your Window installation drive is properly mounted : clic on the Start menu then Computer, then check the drives (in my case, my Virtual CD Drive is the letter E:)
18 - Open CMD tool again, but this time as an Administrator : clic on the Start menu, then in the search bar type CMD, then right-clic on it in the menu and select Run as Administrator
19 - Type C:\IMAGEX.EXE /APPLY E:\sources\install.wim 1 G:\and hit Return Key(remember to change the letters to fit your setup)
there's several spaces in the above command ligne.
You'll see the ImageX Tool for Windows running, with a percentage [ X% ] Applying progress. It'll take 8 to 12 minutes to complete.
20 - Now we'll make the USB Drive Bootable. Type BCDBOOT G:\WINDOWS /S G:
Boot files successfully created
You're Done !!
Now, plug the USB Drive to your Mac. Start it and hold the ALT Key, and choose the Windows USB Drive that you used all along.
Windows will boot, and then will reboot the computer. at the boot chime hold ALT key and again choose the Windows drive to boot from.
Once on the Windows desktop appears, plug your Apple Windows Drivers USB keydrive and install the Bootcamp drivers.
After a while, the installation will be done, you just have to reboot Windows, hold ALT again at startup to choose the Windows drive, and voilà !
You just completed the installation of Windows on an external drive, and you can boot it whenever you want. The best part of this is that you can use your Windows USB Drive only when you want to use Windows, and free all that space from your internal Hard drive.