Before a full OS loads, the system will only display output on a single display. If the laptop is open, the built-in display will be used. If it is closed, one of the external displays will be used. That "Bootmgr is missing" error is occurring in the pre-boot phase before a full OS loads, so the display behavior you are seeing at that point is normal. There is no way to have that message displayed on both displays simultaneously, and I don't think there is a way to have the system use the external display even when the built-in display is available at that point. As for the "Bootmgr is missing" error itself, there are several possible causes for that.
jphughan
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January 21st, 2019 20:00
Before a full OS loads, the system will only display output on a single display. If the laptop is open, the built-in display will be used. If it is closed, one of the external displays will be used. That "Bootmgr is missing" error is occurring in the pre-boot phase before a full OS loads, so the display behavior you are seeing at that point is normal. There is no way to have that message displayed on both displays simultaneously, and I don't think there is a way to have the system use the external display even when the built-in display is available at that point. As for the "Bootmgr is missing" error itself, there are several possible causes for that.