Dual-booting between Mojave and Monterey

Question

Level 1

14 points

Dual-booting between Mojave and Monterey

I have a 2019 iMac, which for many legitimate reasons, I want to keep on Mojave 10.14.6 for the foreseeable future. However, that also raises the problem of not being able to use a few specific apps that demand 10.15 or higher. Ideally, I want to keep Mojave on my very fast internal SSD and use an external SSD to boot into Monterey when required.

However, I have been told that it is not safe to dual boot a Mac system with the older OS on the internal drive and the newer one on an external drive (supposedly due to firmware issues). Is this true, and if so how much of a problem can it be? What dangers will arise? I have never had issues dual booting Macs in the past, but I have generally had the later OS on the internal drive and not the other way around.

I do have an older Macbook from 2015 which has High Sierra and Monterey both on the internal SSD with their own partitions. The only issue I have experienced with that is High Sierra not recognizing or mounting the Monterey partition, but that is actually a good thing for security and has not caused any other problems in daily use. I could go down the path of partitioning the internal SSD on my iMac, but space is at a premium so I would rather have a nice clean install of Monterey on an external Samsung T7.

Posted on Dec 28, 2021 7:22 AM

Reply

Answer 1

Adam Meath Author

Level 1

14 points

Dec 31, 2021 12:55 AM in response to PRP_53

Since I had already installed Monterey on my 2015 MacBook Pro, I used that system to install Monterey onto the external Samsung SSD. It went smoothly, and I was able to boot from the external drive on that MacBook.

I was then able to use that SSD to boot my 2019 iMac, and it appears to work flawlessly. The only minor issue is that when the SSD is plugged into the iMac when running Mojave, it will not recognize the Monterey system partition (nor does it display in the Startup Disk preference pane), but holding down the option key at reboot will work.

I also made sure to disable automatic updates on the SSD Monterey install, which should prevent any unwanted firmware updates being applied to my iMac. I will only run manual updates on the SSD when it is booting from my MacBook Pro (which will be kept at least one incremental release ahead of the SSD).

It seems the reports of failures may be down to one or more of:

Earlier releases of Monterey
Specific hard disks / cables
Macs with T2 chips (my iMac model was the last one without this chip)

Reply

Answer 2

PRP_53

Level 10

85,423 points

Dec 28, 2021 1:51 PM in response to Adam Meath

From what has been reported, at length and in detail, if one attempts to install Monterey to anything other than a Original Apple Drive, be it internal or external - it will fail.

Reason, the EFI Boot ROM included in Monterey is verifying if the Destination Drive is Apple Original or After Market.

The only solution, thus far, is to Install on the Original Apple Drive and allow the EFI Boot ROM to Flash the computer ( not the drive itself ) and then switch out the Apple Drive for the After Market Drive and run the Monterey Install back to Third Party Drive.

Once above is achieved, one could install Monterey to the External Drive with the needed Startup Disk changes applied.

Reply

Answer 3

Adam Meath Author

Level 1

14 points

Dec 28, 2021 9:32 PM in response to PRP_53

Thanks. I was not aware of those reports, given that I have been using Mojave for 3 years and only started contemplating an upgrade in the last few days. I did not realize it was as bad as not being able to install to an external SSD at all.

I do not understand the rationale of enforcing that check on external SSDs on older Intel Macs without T2 chips. If it is about security, my Mac would already be considered exposed on this front, without T2 and other boot related protections. If it is about technical compatibility (or money), my internal SSD was Apple provided and I had no choice in that matter, and Apple does not compete in the external SSD market. I suppose I will get flamed for saying this, but this just seems like a customer-hostile move, born out of either neglect/inattention or a desire to enforce a more rigid conformity in how Macs are used in the Apple Silicon era.

Given that, it appears it not viable to consider anything other than an upgrade of the system on the internal SSD, preceded by a transfer of Mojave onto the T7. Not sure if I actually want to go ahead with that, but will see.

Reply

Answer 4

Dec 29, 2021 12:51 AM in response to PRP_53

> if one attempts to install Monterey to anything other than a Original Apple Drive, be it internal or external - it will fail

Yes, I have seen reports and advice to use the original drive in some Macs to be able to successfully install Monterey.

But on the other hand, I have successfully installed Monterey 12.0.1 (now 12.1) to an external 3TB Seagate HDD on my T2 Mac mini 2018 for testing.

BTW my son dual boots between internal Big Sur and occasionally another small internal Mojave volume to run Lightroom 6.14 on his 2014 MBP (it does not support Monterey).

Reply

Answer 5

PRP_53

Level 10

85,423 points

Dec 29, 2021 1:49 AM in response to Adam Meath

There has been a Process builtin into older versions of macOS called " do not seal " for something like that which was the guard against installing macOS on Non Apple Computers. It has existed for some time.

Apple has since refined this process, it appears, they have included the Original Parts aspect into Monterey.

As colleague @Matti Haveri has mades pointed mention " Anything . . . internal or external - it will fail " .

By Flashing the computer to an Apple Original Drive - that will allow one to install Monterey to and External Drive thereafter

The walls of " The Walled Garden of Apple " have just gotten higher.

Reply

Answer 6

PRP_53

Level 10

85,423 points

Dec 31, 2021 2:24 AM in response to Adam Meath

Excellent work there :-)

Thank you for sharing the finding and perhaps one has fallen onto something others may find useful.

Reply

Dual-booting between Mojave and Monterey

Similar questions