What determines the monitor used to display the Apple logo, etc., at power on?
I have a second monitor with its display rotated 90° to a vertical orientation. That monitor displays the first steps of power on instead of on the main monitor after my upgrade to 10.12.3.
Screen arrangement is correct and the menubar is located on the main monitor in display preferences, so everything straightens out when booting is finally complete. It is just annoying to have to login sideways on the second monitor.
How can I change it back to screens appearing on the main monitor during the initial power on?