Can I install macOS 12 on a separate partition on Mac mini?

mrbl wrote:

Will I have to reformat the entire internal HD to accomplish this OS upgrade?

No, you should not have to.

Is it even permitted to have 2 OSs in the same HD, although they're in separate partitions?

Yes, of course it is. there is no issue with having to Oses on 2 partitions on the same drive outside of the obvious. If the drive dies, both OSes will go with it.

If the answers to the above are both Yes, how should I do this?

You will need to get the macOS 12 installer and create a bootable USB installer, boot the Mac with the installer, and then during installation, simply select the partition you want to install to from the available options.

click here ➜ Create a bootable installer for macOS - Apple Support

Once installed, you can select what drive to boot from, by holding down the option key during boot up. It will bring up the detected bootable drives and prompt you to select one.

click here ➜ Change your Mac startup disk - Apple Support

No, you will not have to reformat the internal HD to accommodate a second OS.

Yes, it's permitted to have 2 (or more) OSs on the same HD.

Before you install macOS 12, please make sure you have a current Time Machine backup of your computer as it is configured today. Should something go sideways you can use that to recover.

How to do it?

(1) - Close all currently running apps and disconnect any peripheral devices and drives.

(2) - Convert the volume "Cuppa'Tune" to APFS format.

Launch Disk Utility and check the format of the volume "Cuppa'Tune". If the volume is currently in APFS format, then skip ahead to (3) .

If the volume is formatted as Mac OS Extended (Journaled), select the volume and then click Edit > Convert to APFS... It may take a few minutes to do the conversion.

(3) -  Download the macOS 12 Monterey installer from the App Store. The installer will deposit an application named "Install macOS Monterey" into the Applications folder. It may also automatically launch that installer, if so then continue on.

(4) - Launch the "Install macOS Monterey" application. When prompted to select a drive or volume to install to, select the "Cuppa'Tune" volume and then follow the prompts through the process to the end.

When the installation is complete, the Mac will reboot into macOS Monterey and you will be prompted to run the Setup Assistant. Make note of the Mac user account name and password you set.

After that you can choose which OS to boot from by pressing and holding the Option key when you restart the computer.

To the OP -

Phil0124 wrote:

[...]

You will need to get the macOS 12 installer and create a bootable USB installer, boot the Mac with the installer, and then during installation, simply select the partition you want to install to from the available options.

click here ➜ Create a bootable installer for macOS - Apple Support

I agree 100% with @Phil0124's guidance to create a bootable installer. Though it isn't strictly necessary in your situation, having a bootable installer on hand can be very useful at times when you may wish/need to reinstall the OS or troubleshoot some problem.

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

"This is not a long-term way to continue using an earlier macOS after upgrading. Later macOS versions can install changes designed to keep your Mac secure, and these changes can affect your computer's ability to continue using a significantly earlier version."

I don't recall that being an issue with versions of macOS before Catalina / Big Sur, but with all of the under the hood security changes Apple has been making in recent years, who knows?

mrbl wrote:

Of course, if things went horribly wrong, I'd still have a a "blank slate" of a CPU, in which the original OS could be recovered online from Apple, with the rest of the content being restored from TM.

Many people have had a lot of problems getting High Sierra. I suggest seeing if you can download the High Sierra installer & build a bootable installer flash drive before doing anything else with that mini.

How to download and install macOS - Apple Support

Create a bootable installer for macOS - Apple Support

Alternatively, clone your existing drive to an external drive. I use CarbonCopyCloner to make bootable backups of my old systems. But I still think it's preferable to have a bootable installer at the ready.

mrbl wrote:

I'd like to install OS 12.x (last latest available for the 7,1) on this empty partition.

I want to hang onto OS 10.13 as it's the only machine in the house for legacy apps.

Although technically possible to install Monterey on the unused partition, I am of the opinion that it's not a good idea to do it because you want to preserve use of those legacy apps (presumably 32-bit).

Monterey is a very, very different OS compared to High Sierra and the installer will also update the firmware in your 2014 Mini. Things *can* go wrong and there is at least *some* risk that it will not be recoverable even with a TM backup. I would even be cautious about installing Monterey on an external drive in this case, but would consider doing it that way before attempting an install on the 3rd partition of the internal drive.

Suggesting as an alternative ... keep the 2014 mini as-is. Get a refurbished 2018 Mac mini (capable of running Mojave through Sequoia) from OWC (price range $279 & up). If you installed Sequoia on it you would be up to date and have 2-3 years of future system updates to look forward to.

mrbl wrote:

Very good advice. If OS 12 were installed on an external drive, would the firmware necessarily be updated for the CPU that the external was using for the installation? In that case, the installation on an external would be just as liable for firmware conflicts, as the internal partition installation.

Yes. Any installation of Monterey will check the firmware, and attempt to update it if it is old. I don't think that it matters whether the installation is internal or external.

This is why, on many older Mac laptops, you should not replace an internal Apple-factory SSD with a third-party SSD before first installing Monterey successfully somewhere. If the firmware updater sees a non-Apple SSD, it will reportedly fail, and the Monterey installation will also reportedly fail. Only after the firmware is up-to-date is installing Monterey with an internal third-party SSD in place possible.

Servant of Cats wrote:

Yes. Any installation of Monterey will check the firmware, and attempt to update it if it is old. I don't think that it matters whether the installation is internal or external.

That's correct. It doesn't matter whether the installation is internal or external. The firmware update is the one remaining reason I still haven't upgraded my 2 Macs running High Sierra even though the 'upgrades' would be installed on external drives. I'd like to have dual-boot capability but those Macs have my 32-bit apps on them and I need to keep them viable. It's all about risk management.

That said, I do have a spare 2014 mini running High Sierra on which I plan to install Monterey on an external SSD in the next month or two. It will be revealing to see how it goes and I'll be better informed then. If successful my plan is to replace the internal drive with the newly-installed SSD. But I haven't done it yet.

The 2014 mini was the media server for my home theater. I just replaced it with a 2018 mini for longevity and so we can watch 4K video.

¡De nada!