Using windows from bootcamp on external disk

I am using bootcamp on an external drive due to the default hard drive capacity being insufficient. However, when using the window from bootcamp feature, I get an EFI error and cannot boot in to install. Is there any way to fix this problem because I have purchased a permanent license to parallels. It's a pity that the emulator mode makes Windows run quite slowly on my computer. It's impossible to install Windows twice on my computer because it's too cumbersome.