I was able to install Windows 8.1 on an external SSD TB drive via bootcamp on a 2013 iMac, however after I went back to OSX, I am unable to load up windows. I tried restarting pressing option and going to the lacie drive, but all it wants to do is reinstall windows. Anyone have experience with this?
In my case, I didn't use Boot Camp Assistant, I used my own method and managed to get it running on either USB or Thunderbolt, booting in UEFI.
Connect external drive to Windows VM. You must have a Windows VM in VMware/Parallels.
What you need:
install.wim file (obtain this from your Windows ISO)
Open elevated cmd.exe
Note: All commands aren't case sensitive, including pathway to files.
Type diskpart
Type list disk
Take note of the disk you want to select
Type select disk 1 (if your disk is Disk 1)
Type clean
Type convert gpt
Type create partition EFI size=100
Type format quick fs=fat32 label=EFI
Type assign letter=S
Type create partition primary
Type format fs=ntfs quick label=W2G (or any other name you wish for label)
Type assign letter=E
Type exit
Open up File Explorer. In your C drive, create a new folder named WIN2GO.
Put the install.wim file in this folder
Back in cmd.exe:
Type dism /apply-image /imagefile:C:\WIN2GO\install.wim /index:1 /applydir:E:\ (this process will take quite a while)
Type E:\Windows\System32\bcdboot E:\Windows /s S: /f UEFI
Restart your entire Mac. After the chime, hold down Option and when prompted to select your boot drive, select EFI Boot.
Proceed installation normally.
After installation, install Boot Camp drivers.