I have two work-flows, one uses Mountain Lion, the other uses Mojave. So ideally I would like to be able to easily switch between the two boot drives I have whenever I need to.
When I have booted from Mojave, I can choose System Preferences -> Startup disk, and then my Mountain Lion drive is there, and I can select it and boot from it.
However, when I boot from Mountain Lion, it says "the disk you inserted was unreadable" and there is no option to choose it from the System Preferences -> Startup disk.
Many resources online say "hold down option when booting and select your boot drive", but this does not seem to work for me. When I hold option down, it does not present any options for drives and just continues to boot Mountain Lion... How can I do this without the annoyance of having to physically remove the drive I don't want to boot?