Booting Windows from external Hard Drive?

I have searched the whole forum and couldn’t find anyone having the same problem with all the specifics in mind.

My laptop uses Linux exclusively. I wanted to have an option to run Windows every now and then, so I have installed Windows on my external SSD. Before installing I have switched drives and inserted an empty brand new SSD in place of my hard drive with Linux. I have installed Windows 10 (legal copy from Microsoft website with no key) on that drive and everything was working fine until I removed the SSD from my laptop and tried to boot it as an external hard drive. It troubleshoots my PC, eventually giving me an option to restart the computer. After restarting exactly the same thing.