Ir won’t touch the MBR because your problem doesn’t have anything to do with an MBR. The MBR is only used for booting in legacy BIOS mode. UEFI systems that boot in native EFI mode use the information in the EFI system partition for booting. 
Ir won’t touch the MBR because your problem doesn’t have anything to do with an MBR. The MBR is only used for booting in legacy BIOS mode. UEFI systems that boot in native EFI mode use the information in the EFI system partition for booting.