Thank you so much for warning me: I have the same model (2014 MBP 15"), and would very likely have tested booting an external drive with another OS, not expecti

Thank you so much for warning me: I have the same model (2014 MBP 15"), and would very likely have tested booting an external drive with another OS, not expecting it to cause harm to my hardware. Maybe this will help; the instructions are for Yosemite though. https://forums.macrumors.com/threads/guide-how-to-get-back-old-efi-after-downgrade-from-yosemite.1803934/ https://forums.macrumors.com/threads/guide-how-to-get-back-o...