I have High Sierra installed on a partition of my MacBook Pro’s internal SSD, and Mojave installed on a partition of an external drive. If I have selected the High Sierra boot volume in Startup Disk and reboot from Mojave, then there is a chime, the screen goes black, and nothing happens. If I reboot with the option key pressed, then the only options are to boot from the Mojave volume, or a network volume. So far, the only way I’ve been able to get back to High Sierra is to shut down, physically disconnect the drive with Mojave, and then start up. Is there a better way to deal with this?