Dual Boot Monterey and Yosemite on Mac Pro Late 2013 ?

• 10.14.2: 127.0.0.0.0 
• 10.14.4: 130.0.0.0.0 - removes requirement for Apple SSDs to perform firmware upgrade
• 10.14.6: 131.0.0.0.0 
• 10.15.3: 134.0.0.0.0
• 10.15.4: 135.0.0.0.0
• 10.15.5: 136.0.0.0.0
• 10.15.6: 137.0.0.0.0
• 10.11.1: 426.0.0.0.0
• 12.4 430.120.6.0.0 

you don't need to install onto the Internal SSD to get a fr=irmware update -- you just need to insytll on that computer. The firmware is kept in private store on the processor card, NOT on any drive.

The old way (for older Macs) was that a Firmware Updater (micro-system) is momentarily placed on the proposed install drive (which after version 130.0.0.0.0 should be ANY drive) and your Mac would then Set Startup to that drive, boot from it, you hold the startup button and it would install new firmware.

Previously the updates were distributed separately from the OS , but in 10.13+. they have been distributed with OS.

from:

https://blog.greggant.com/posts/2019/05/07/the-definitive-mac-pro-2013-trashcan-guide.html#tfirmware

not very much less complex when viewed from Disk Utility, except in my machine a PCIe slot card is always seen as external. The physical DEVICE is the SHGP31.10... entry:

when you click the Partition tab, you are offered the ability to add a Volume (inside the APFS Volume Group) or add an additional Partition (essentially a free--standing partition) which is what is required because older MacOS can not see inside an APFS volume group at boot-time.

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Remember, there is a certain amount of RISK involved in partitioning a drive with data already on it. The risk is that Disk Utility will get confused and re-write the partition table WRONG, essentially deleting all existing Volumes and Partitions. You MUST have adequate recent backups to even think of going down this route.

<< I'd read on eclecticlight.co that firmware updates that are more than three generations away from an older OS can cause kernel problems. >>

... that might apply to installing MacOS too far forward from the current firmware, but that overly-broad generalization does not apply to backward-compatibility on the Mac Pro, whose firmware upgrades have been seen as almost flawlessly backward-compatible.

Rather than attempting to cram everything onto your internal drive and fiddle around with multi-OS partitioning, I stand by my recommendation to use an external drive for your older MacOS. It just works.

High Sierra by default will format the boot SSD Volume to APFS format.

But two versions later in Catalina, APFS Volume Groups are the standard. And the version after that in Big Sur, the locked system volume is introduced.

here is a glimpse at the complexity involved in an Internal (PCIe slot) APFS Container disk in my Mac Pro 5,1 running Big Sur:

/dev/disk2 (external, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk2

1: EFI ⁨EFI⁩ 209.7 MB disk2s1

2: Apple_APFS ⁨Container disk4⁩ 1000.0 GB disk2s2

/dev/disk4 (synthesized):

#: TYPE NAME SIZE IDENTIFIER

0: APFS Container Scheme - +1000.0 GB disk4

Physical Store disk2s2

1: APFS Volume ⁨M11-P31 - Data⁩ 487.3 GB disk4s1

2: APFS Volume ⁨Preboot⁩ 381.7 MB disk4s2

3: APFS Volume ⁨Recovery⁩ 620.4 MB disk4s3

4: APFS Volume ⁨VM⁩ 1.1 MB disk4s4

5: APFS Volume ⁨M11-P31⁩ 15.3 GB disk4s5

6: APFS Snapshot ⁨com.apple.os.update-...⁩ 15.3 GB disk4s5s1

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What you are proposing, on TOP of that, is to carve out a HFS+ partition to put High Sierra or older onto the same drive.

Because of my pre-conceived notions, I assumed you would install the more advanced version of MacOS like Monterey on the INTERNAL drive. That might produce a firmware upgrade, which might make recognizing a Monterey install on an external drive easier.

¿Can you Re-install Monterey on the external drive using the Mac Pro itself?

As I mentioned, the firmware is installed in a private store on the processor card, not on any drive.

Spanning. What did you end up doing? Like you, I’ve scoured the web for the exact same issue (with the same reasons) and have not found a solution that works. I’m preparing to try plan #4 or 5. Taking an external drive formated in HFS+ and partitioned into 3 (just for the heck of it) on an El Capitan machine. First partition has a successful Yosemite boot (tested).

Next to do: The 2nd partition is going to be for Monterey which I will format as APFS via a Mojave machine. Then using BelightSoft steps or their software, I’m planning to clone a current Monterey drive to the external drive 2nd partition and test that it boots. IF both boots are successful, then I will duplicate that on the internal drive.

Here is why I’m testing this first. I originally loaded Monterey on an external SSD so as not to disrupt my internal Yosemite drive. It took awhile, and multiple OSs to get there, but I tested the boot into monterey before swapping the drives inside my mac. Then I partitioned the Monterey drive into 2 and formated the 2nd partition to be HFS+. However, I could not even read the 2nd partition if I booted into Yosemite (now the external drive since the swap). So APFS formated drives do something to partitions that are formated as HFS+ by an APFS MacOS that make them unreadable by older MacOSs. I understand APFS not being seen by older Mac OSs but HFS+? I’ve tried multiple other things with terrific failure. Will not write all I’ve done here.

It’s taken far too long to research and even get to this point and has seriously disrupted my work flow. So I’m hoping for any insights and I’ll also report here other key findings. I’m trying to avoid carrying an extra drive when traveling, hence dual boot partition on the internal drive.

Thanks.

either of the external drives may be easier to live with:

approx actual size, about 1/4-in thick

Apple firmware is generally backward-compatible.

You can place either of those macOS installs on an external drive, and not have to deal with any complex issues at all.

what version of firmware are you running?

 menu > About this Mac > (System report) ...

system version number might look like 136.0.0.0.0