How to allow start up from ext disk? [Resolved!]

I'm trying to allow my MM2018 to start up from an external disk (to test if the initial clone works).

Following this: https://support.apple.com/en-gb/HT208198#open
I get stuck at #3 (When you're asked to authenticate, click Enter macOS Password, then choose an administrator account and enter its password.)

After clicking 'Enter macOS Password' on the 'Authentication Needed' window, my name's already filled in the 'User' area, but it doesn't accept my Password? I'm the only user and 'Users & groups' show me as Administrator. What to do?

Any help much appreciated!

 

As I understand that should be done after a T2-chip protected Mac, like my MM2018, has been set to allow starting up from an external disk in the first place - see the first link in my first post.

--- Post Merged, Dec 2, 2018 ---

Update: the suggestion to torn on and then turn off FileVault (https://discussions.apple.com/thread/8486592) doesn't work either (for me).

--- Post Merged, Dec 2, 2018 ---

Like I said earlier, when you do what you suggest, on a T2 protected Mac you get



And it's in that process (Command-R > macOS recovery > Startup Security Utility that my Admin password is not accepted.

--- Post Merged, Dec 2, 2018 ---

Just to add: my Admin password is the original password I chose when setting up the Mac mini and has not been changed.

 

Thanks for that. Sounds pretty drastic, but could give it a try although it does not mention Mojave...

 

Update: I found the 'solution' - my (French) keyboard was apparently not recognised in the Command -R recovery procedure, so retyping my admin password as if on a US/UK keyboard worked - phew!

 

Indeed, but how/where?

Edit: found it!