Windows just doesn't like to run from removable devices period. What you have to do is mark the USb drive as non-removable. How you do this varies with the drive, usb2, usb3, controller type... all make a difference. There used to be a more generic method but I'm not sure it's still valid and I'm lacking time to test esp when there exist better methods to recover from a failed drive.
As for why you are doing this, it doesn't make sense to me. Why waste a whole drive with 1 copy of your windows to rescue your current drive when you could have several copies, of different points in time, by storing images of your drive instead. Macrium restores images just as fast as cloning a drive back to the replacement drive.
If you are just going to set the drive aside until you need it, then just clone to it periodically and install it in place of the system drive when it fails.