How to install MOJAVE and VENTURA on same drive? (errors)

[Dochartaigh]

I have a slow 2017 MacBook 12-inch (travel computer for light Camera RAW edits). I want to do a test so I can compare Mojave (my previous go-to which I think seemed almost snappy on this older computer) to Ventura (which is NOT snappy at all - but want to see how much worse it really is)...

...but no matter in which order I install both these OS's I get warnings/errors about the SSD partition/formatting type. Sorry, but I can't for the life for me remember the exact errors... something about the SSD format type and how it 'might not be compatible with all features' of the new OS or something to that effect. This error pops-up every time I boot the computer (maybe only in Mojave - I forget). Seems to still work fine but always worrying...

For how I've been trying this: I use a bootable USB, boot into safe mode and use Disk Utility to totally delete all SSD partition(s). Then I've tried it both ways: tried Mojave 1st then Ventura 2nd, and Ventura 1st then Mojave 2nd). Also believe Ventura (and all the way from Catalina?) has a thing where you don't need a totally separate partition when you have multiple macOS's on the computer and it'll share the home/user folders or something like that? But I think I tried Ventura-then-Mojave that way; and also tried (both ways) totally separate partitions (each with their own individual user folder) and I was STILL getting the weird error when I booted into Mojave... Also don't know if formatting APFS vs Mac OS Extended (they changed between Mojave and all newer OS's, right?) would slow down one OS vs. the other, which I would not want that to happen of course...

What is the proper way to do this? (will need directions on which OS to install first, what APFS/GUID/etc. format type for the SSD, if I need totally separate partition –or not– when i install the 2nd -- format type for that if applicable, etc. please). Tried this last maybe 5 months ago... and tried it literally 5-6 times in varying OS-install-order and still could never get it right... (I currently have Ventura-only installed on it).

 

[StellarVixen]

Use more than one version of macOS on Mac - Apple Support

If you install a compatible macOS on an additional internal or external storage volume, your Mac can start up from either volume.

support.apple.com

Did you try it as it was instructed there?

 

[Dochartaigh]

Use more than one version of macOS on Mac - Apple Support

If you install a compatible macOS on an additional internal or external storage volume, your Mac can start up from either volume.

support.apple.com

Did you try it as it was instructed there?

Yes, believe that's the method when you have two separate partitions (no shared User folder and such), and was STILL getting that error/warning when I booted into Mojave (again, this was a while ago but I'm pretty darn positive as I tried it multiple times... and was actually doing this on two computers then as well - had a 2018 Macbook Pro 15" I've since sold).

*edit: might have also been because I formatted the drive a different way than I should of (maybe for the Mojave install? -- which is why I'm asking for exact instructions so I can try this again).

 

[Grumpus]

There are three ways to approach the problem - installing Mojave into a new APFS volume, as described in the link provided above by @StellarVixen, or partitioning the drive and installing Mojave into the new partition, or installing Mojave onto a USB-connected SSD. All will result in warnings when you boot into Mojave.

Ventura uses a newer version of APFS which provides features that Mojave doesn't know anything about. When you boot into Mojave, it will automatically try to mount the Ventura "disk" resulting in the warnings. You can just ignore the warnings and eject the Ventura disk when you're running Mojave (right-click, select Eject).

 

[Dochartaigh]

There are three ways to approach the problem - installing Mojave into a new APFS volume, as described in the link provided above by @StellarVixen, or partitioning the drive and installing Mojave into the new partition, or installing Mojave onto a USB-connected SSD. All will result in warnings when you boot into Mojave.

Ventura uses a newer version of APFS which provides features that Mojave doesn't know anything about. When you boot into Mojave, it will automatically try to mount the Ventura "disk" resulting in the warnings. You can just ignore the warnings and eject the Ventura disk when you're running Mojave (right-click, select Eject).

Ahh! Thanks. So that solves the mystery: Mojave trying to mount the Ventura disk gives me that error (I get it immediately from first boot as well - when I'm still initially setting up the Mac's keyboard and such).

So... now that I already have Ventura installed, is the above directions still correct? Those are how to install a newer OS from an older one, when I want to go the opposite way (install an older OS from a newer one). -- just making sure as I couldn't even make a Mojave bootable USB in Ventura... they removed that ability for some weird reason.

 

[Grumpus]

So... now that I already have Ventura installed, is the above directions still correct? Those are how to install a newer OS from an older one, when I want to go the opposite way (install an older OS from a newer one). -- just making sure as I couldn't even make a Mojave bootable USB in Ventura... they removed that ability for some weird reason.

I'm not sure I followed that. The more usual situation may be installing a newer OS into a volume created by an older OS, but that's just a guess. In that case there may be no warning messages. In any event, the very first thing you should do is to back up anything you don't want to lose.

You should be able to download Mojave from Ventura. Open a terminal window and type:

softwareupdate --list-full-installers

You should see a list of available macOS installers, with Mojave probably at the end of the list. To download the Mojave 10.14.6 installer, you'd type:

softwareupdate --fetch-full-installer --full-installer-version 10.14.6

It takes a while, but when complete the Mojave installer will be in /Applications. You can then follow the instructions given here to create a bootable USB installer. I would be very surprised if that doesn't work under Ventura, but I've been surprised before, once or twice.

 

[Dochartaigh]

I'm not sure I followed that. The more usual situation may be installing a newer OS into a volume created by an older OS, but that's just a guess.

Was just asking because I am indeed doing the opposite - so wanted to see if there was anything special I needed to do. Actually already did it since I posted so all seems to be well so far (besides the 'new' Mojave not seeing my Bootcamp partition/drive I created from Ventura... and when I run Bootcamp installer on Mojave it wants to create a new one when it's already there... anywhoo...).

You can then follow the instructions given here to create a bootable USB installer. I would be very surprised if that doesn't work under Ventura, but I've been surprised before, once or twice.

Yup, that is exactly it. Seems to be a thing that Ventura is unable to do. Found this post, with a bunch of other posts linking to it as the only answer it seems... no matter anyway - I just used my 14 year old computer to make it instead - was just wondering why I couldn't do it this time on Ventura when I've ALWAYS been able to make it before.

 

[RoyalScam]

Thread back from the dead... because I myself added a new Ventura volume alongside my Mojave. I too get the "Incompatible" error when I boot into Mojave, but I now know it's safe to ignore it.

Question though...why am I not seeing the Ventura drive in Startup Disk prefs on Mojave or the Mojave drive in Startup Disk prefs on Ventura? Is holding down "Option" my only, er, option to choose which to boot into? I use my MBP in Clamshell mode, and the Option key isn't working on boot-up from my external keyboard.

(I already have Startup Security Utility set to Medium Security and "Allow booting...")

 

[galad]

macOS 11 introduce signed system volumes. 10.14 has got no idea of that, and so it can't read it or select it as a boot option.

 

[RoyalScam]

macOS 11 introduce signed system volumes. 10.14 has got no idea of that, and so it can't read it or select it as a boot option.

Understood now. Thanks.

Any ideas on how to get my wired Mac keyboard to work when holding down its Option key while the MBP is in Clamshell mode so I don't have to constantly use the MBP's keyboard every time I reboot?

 

[K two]

Understood now. Thanks.

Any ideas on how to get my wired Mac keyboard to work when holding down its Option key while the MBP is in Clamshell mode so I don't have to constantly use the MBP's keyboard every time I reboot?