Dual Boot, Mojave and High Sierra Much Easier Before Upgrade

[danallen]

It seems like these are great instructions for setting up dual boot with Mojave and High Sierra, except there is a "does not compute" portion in the process.

First it says:
"If you are running High Sierra on an SSD or have already installed Mojave this process is much easier because your Mac will be using the new Apple File System APFS."

Turns out that statement is partly incorrect.

The first clue of the error comes where the same article says:
Download the installer for the OS you want (it will have to be a newer version than the one you are running currently - if it isn't then the process will be more complicated and you should read this: How to downgrade to an older version of macOS.

What the hey? The problem is setting up the dual boot is easy if you are running High Sierra and want to add a bootable installation of Mojave. Once you you upgrade to Mojave, setting up the dual boot becomes a lengthy process.

This was my question, can I just install High Sierra on a volume I just made on the same drive as the one where Mojave is running?

The answer turned out to be no.

 

[danallen]

Here was my question, can I just install High Sierra on a volume I just made on the same drive as the one where Mojave is running?

Looks to me the answer is NO. When trying to open the High Sierra installer on a Mojave installation (intending to install High Sierra in separate volume), this is the result:

I guess I am going to forget about dual boot with Mojave and High Sierra. I think the instructions I found are in error where it says setting up dual boot when running Mojave is easy. It is easy to setup the dual boot with Mojave and High Sierra if your system is running on High Sierra and have not upgraded to Mojave. However, contrary to this article, once you upgrade to Mojave without the dual boot then it is too late to add dual boot with High Sierra without going through a lot of steps.