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I use rEFInd as a bootloader. It will find any Windows (or other OS) on any attached drive (internal or external) and give you the option to boot from it. I have Windows on SD card and it even finds that and will boot (but it runs like a snail)...
Alternatively if you run bcdboot.exe the standard Windows loader will give you the option to boot from USB drive and just fail if you chose that option and it isn't plugged in. usb - How to run Windows 10 from an external hard drive? - Super User