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!