There is also nvram (UEFI menu) setting. Until recently, Calamares replaced the main disk default boot entry with the installing OS option. This is physically existing on the internal drive. Unless EnOS installer is bypassing this behavior with some workaround, when the external is disconnected on boot, there may be an issue, depending on UEFI Vendor’s implementation.
I am not aware if this was fixed upstream Calamares. Does anyone know?