I am running Catalina as my primary OS but am finding that I'd still like to run some 32-bit apps. Is it possible to create a new AFFS volume, install Mojave and simply run a dual-boot system?
Things that did not work:
I have created a new APFS volume, but I cannot figure how to make it bootable.
I was able to download an official build of Mojave but cannot get to the install screen because of an error message telling me This copy of the “Install macOS” application is too old to be opened on this version of macOS.
Best Answer
Yes, this is definitely possible. I just created this exact setup with macOS Catalina (10.15.1) as the "main" OS, and a separate macOS Mojave (10.14.6) installation in an APFS Volume (same disk, same container).
Follow these steps:
Step 1
Download "Install macOS Mojave" archive
(from https://www.macworld.co.uk/how-to/mac-software/downgrade-macos-3581872/)
Step 2
Create APFS volume
Mojave
in "Name" (leave "Format" as "APFS" - I also did not specify any size options)Step 3
Create bootable installation media for Mojave
Follow the official instructions at How to create a bootable installer for macOS
In short summarized form:
Format a min 16GB USB stick using Mac OS Extended, call it
boot
, then run:Replace
/Applications
with~/Downloads
in case the installer is still in your Downloads folder.Step 4
Boot from install media, and install to
Mojave
volumeMojave
volume when asked where to installOnce the installation is finished, you can use the "Startup Disk" program to set you main Catalina installation as the default. When you want to boot Mojave, just restart the computer, hold ⌥, and select "Mojave".