I have followed this guide: https://9to5mac.com/2017/08/31/how-windows-10-mac-boot-camp-external-drive-video/
When I select EFI Boot from my Samsung M3 portable drive it reboots to the boot recovery assistant and asks for my pw to unlock the macOS disk.
This is the second HD I've tried this on, both to the same effect.
The drive is partitioned 75/25 Time Machine / Windows.
Any help is appreciated