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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?

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One possible explanation is that Mojave is on an APFS volume, which Mountain Lion doesn't know about. It's possible that your Mac's firmware also needs updating if the APFS volume is not being shown in the Boot Manager thing (Option at Start).

Seemingly, you have managed to get Mojave booted somehow, so it's not an incompatibility/lack of support for your hardware.

I currently run Mountain Lion in a Parallels VM, on my Mac running Mojave. You might want to consider that, as an alternative to boot-switching.

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