Two versions of Macos on a single Mac

If your mac came preinstalled with Catalina you cannot install an older OS on it.

If your mac came preinstalled with Mojave or older you can install Mojave on the

partition you created.

Open Safari and click on the link below, Firefox will not work.

How to upgrade to macOS Mojave - Apple Support

You need to create a Mojave bootable installer, boot from it, then install Mojave to the new volume you created.

How to create a bootable installer for macOS - Apple Support

And if you want to use the same internal SSD APFS, you must partition it "old style" because AFAIR the new "add volume" that Disk Utility recommends for APFS does not support dual booting.

I'd use an external disk for an occasional Mojave boot.

Or virtualize El Capitan.

Matti Haveri wrote:

And if you want to use the same internal SSD APFS, you must partition it "old style" because AFAIR the new "add volume" that Disk Utility recommends for APFS does not support dual booting.

That's incorrect. I have an iMac with Catalina on one volume and Mojave on another. No partitioning.

And if you want to use the same internal SSD APFS, you must partition it "old style" because AFAIR the new "add volume" that Disk Utility recommends for APFS does not support dual booting.

Sure does. Works for High Sierra through Catalina. However, to have High Sierra in the mix, you must install that first.

Good to know that partitioning APFS as volumes should work for booting. A few months ago I unsuccessfully tried it and reverted to the old scheme. But maybe there was another glitch that caused it then.

Re-trying to use APFS volume groups for dual boot reminded me why I reverted to the old partition scheme. Long story short:

"It is not possible to bless another system on the same APFS volume group".

My setup for this is somewhat special so here are the details for those who are interested:

I use an external disk as a test bed for Catalina and occasionally also Mojave. It is partitioned as 1. Catalina-clean (install), 2. Catalina-test, 3. Mojave-clean and 4. Mojave test. I can quickly reset the test partitions to a clean install state by cloning the clean partition to it with Carbon Copy Cloner (actually I usually clone it from a locked CCC disk image).

OK. Making APFS volume groups was very handy and unlike old partitions, as a great bonus, it left unused space on the small 320GB HD shared between all those volumes (it is an old slow spinning 5400 SATA HDD kannibalized from an old Asus laptop. I am cheap).

But where this seems to fall apart is that when I am booted from the Catalina-clean, CCC refuses to completely clone it (or from a disk image saved on another external disk) to the the other APFS volume (Catalina-test) because:

"It is not possible to bless another system on the same APFS volume group". In Mojave CCC seems to successfully clone all files but the blessing must be done while booted from elsewhere. But CCC seems to alert that much of Catalina content can't be cloned then.

Anyway, maybe I continue with APFS volume groups because it seems to handily share unused space for all volumes. Maybe I could tweak the APFS volume groups on that same disk so I can boot from another group to do the blessing? Or add an old-style extra partition for Catalina so I can bless the APFS Catalina volumes (or if that fails, use some other spare disk or flash drive to bless the Catalina volumes (this is my one and only Catalina install, Mojave being my main system).

Creating a separate Container disk for the other Catalina install on the same device via the old-style partitioning scheme fixed the problem. So now I have two Catalina installs that can bless each other on the same device.

In retrospect, the CCC alert DID tell me this: "When Disk Utility tries to discourage you from doing this, by preselecting "Add Volume" click the "Partition" button ... Click the "+" button to add a second APFS-formatted partition on the startup disk".

The problem was that bootable APFS volumes in the same container will all share the same Preboot volume. They each get their own payload folder, but the volume itself is shared. The "bless" problem arises when System Integrity Protection prevents any changes to the Preboot volume.

If you are entitle to it (real Apple hardware), you can have all possible versions of macOS installed on your computer.

I Have 10.14, 10.15, and 11.0 installed at same time.

That is a problem of CCC. The installer can do it.

Again, that is a ccc cloning problem and has nothing to do with multi-boot on the volumes in a container.