Can you install macOS Ventura on an separate volume? (Apple Silicon)

Yeah, I just got bitten by the same issue! I set the installer to download overnight (fairly slow internet here) and woke up to see my MBP had been upgraded automatically! Why on earth has this process changed??

I can't even see how to downgrade back to Monterey.

I've removed the machine from the Beta access programme and checked software update, but it's not offering 12.4. Using the "Download" button on developer.apple.com/download simply opens the App Store on the page for Monterey. When I click the "Get" button, it just takes me straight back into Software Update where it shows "Your Mac is up to date".

I do have a backup (thanks CCC!)...but of course, they're not bootable backups these days, so I booted into Recovery mode, to see that the only option there is to "Reinstall MacOS Ventura". Talk about unlucky number 13!

I don't have any particular issue with Ventura, at least not yet anyway, but I certainly didn't want it to usurp my main laptop.

Noooo! I thought I was on to a winner with Internet Recovery mode. It gave me the option to "Reinstall MacOS Monterey", but when I unlocked my internal Hard Drive and selected it as the installation destination, I got an error message saying:

The operation couldn't be completed. (com.apple.BuildInfo.preflight.error error 22.)

I had this issue with a borked install of the Ventura beta overtop of Monterey while trying to install it on a side partition.

I had to actually use another MacBook pro to restore MacBook air to use again.

Do you have a recovery USB? use that instead of internet recovery. If you do not - use another Mac to revive and then restore the 'afflicted' Mac using Apple Configurator.

Internet recovery will be broken for you at the moment as it was for me so you'll need to use either a bootable USB or another Mac to restore your computer.

I just spent the last 24hrs on this. Hope this helps

You can just download the latest Monterey full installer here

That sounds like what I'm attempting now. I used an old Mac Mini to install Monterey onto my existing data-volume-only backup.

I've now managed to boot my MBP from that. The Monterey installer informed me that I can't downgrade the OS on my internal drive, so I've erased it and am now using CCC to perform a data only backup/restore to the internal drive. When that's finished, I'll (hopefully) use the Monterey installer to install macOS 12 back onto my MBP's internal hard drive.

This has been an enormous waste of almost two days of my time.

One thing that still concerns me is that I'm using Documents and Desktop in iCloud Drive to share files between that laptop and my desktop Mac. When I finally get the machine restored from backup and macOS 12 installed again, will it attempt to upload everything to iCloud again? The backup is about a week older than what's on my desktop computer. It looks like it's trying to do exactly that just now, whilst booted from the external SSD - I turned off WiFi and killed the bird process, which has stopped the attempt for now.

OK, so after two days, I'm finally back up and running. The procedure I described above worked almost perfectly. The only two snags were the attempted upload to iCloud Drive again, and the fact that CCC restored my Documents folder to ~/Documents (essentially a duplicate of what was in iCloud Drive).

I'll wait until the next beta before trying macOS 13 again, and hopefully I'll be given the option of where to install it as per previous years.

ME2