How to install Mojave on separate volume of Catalina drive

You didn't mention what version of macOS your iMac came with. If it came with Mojave, you need to create a bootable installer of Mojave, boot from it, then install Mojave to the volume you created. If you have a 2020 iMac, it won't boot from Mojave.

How to create a bootable installer for macOS - Apple Support

If you can't download Mojave installer because you are running Catalina, I'm not sure what to do.

I'm wondering if you could boot into Internet Recovery using cmd-shift-opt-R and install the OS that shipped with your Mac, boot from that, then install Mojave (if it didn't ship with Mojave).

I haven't tried that, though.

Your Mac would have had to ship with High Sierra or later to make this work, though, as the older OSes can't handle APFS.

I attempted it, but my Mac Mini shipped with Mountain Lion. I could see all the other disks, but not the APFS volumes, so I think it would work.

If I remember correctly, Apple has links for such downloads directly on their support web site. I don' remember exactly where it is though. This works, unlike trying to do so via the Apple Store. I hope this helps.

Ken Parsons wrote:

And that brings me back to my original problem: I can't create the bootable installer, because I can't download the installer! The Macworld article glossed over this very same issue.

I see. The message you quoted in your first post…

"This copy of the "install macOS" application is too old to be opened on this version of mac OS."

is what you would see after the Mojave installer has downloaded and tries to open. You should see a file in the Applications folder "Install macOS Mojave". That's what you would use to create the bootable installer. If it's not there and you are using Safari to download the file, uncheck "Open "safe" files from Safari > Preferences > General". Then download once more.

That's fine. As long as it didn't come with Catalina. Create the USB Mojave bootable installer and you'll be set.

I suggest that you install it on separate partition in lieu of a separate volume, otherwise Mojave may not be bootable.

If your Mac shipped with High Sierra or later, try the Internet Recovery option.

Maybe a temporary server issue.

“otherwise Mojave may not be bootable.”

Not sure why it wouldn’t boot from another volume in the container. If it couldn’t do that, Recovery wouldn’t boot.

I've got the problem solved now —thanks to everyone for their helpful suggestions!

Can you describe what you did to provide a solution for others seeking the same thing.