iMac Intel: MacOS Sequoia and Sonoma cannot coexist ?

Deber wrote:

Since the iMac has the Apple T2 Security Chip, I have set Medium Security with the Startup Security Utility and 'Allow booting from external or removable media'.

It has been a while since I dual booted a T2 Mac with two versions of macOS......do you need to configure it with "Medium" security? I would leave this at the default (High) unless it is absolutely necessary to change it.

My problem arises when I switch the startup disk back to Sonoma, as the internal disk with Sonoma always appears corrupted and shows the message "The version of macOS on the selected disk needs to be reinstalled".

I don't recall ever seeing this error message on an Intel Mac although I have seen it numerous times on M-series Macs (rarely have a clue as to why though).

I'm thinking this may be more related to the M-series Mac's concept of ownership which may somehow been somewhat backported to the Intel Macs. Are the two operating systems using the same exact names for the macOS user accounts? And were they created in the same order so that they have the same UserIDs? The T2 security chip some how retains or associates these and I can see having two operating systems (could both be Sonoma even) could get the T2 security chip confused. I've seen the T2 security chip get confused with just a single OS with multiple user accounts.

Perhaps try performing a DFU Firmware Revive which theoretically should not affect any data on the internal SSD. However this will likely reset the firmware security settings, so you will need to use the Startup Security Utility again in order to boot from your external drive again. Unfortunately the DFU Firmware Restore does require access to another Mac currently running macOS 15.x Sequoia (once macOS 26 Tahoe is released, that will be the OS requirement for the host Mac for this procedure). Or better yet perform the DFU Firmware Restore which will also reset the internal SSD as well since you are performing a clean install of macOS daily anyway (make sure you can boot into Internet Recovery Mode since there is no USB booting until you reinstall macOS to the internal SSD through Internet Recovery Mode).

How to revive or restore Mac firmware - Apple Support

In 'Recovery Mode' Disk Utility doesn't report any problems with the internal disk, but I can no longer boot from it.

Don't trust the First Aid summary screen as it will lie about the results. You must manually review the First Aid log by clicking "Show Details".

If I only try to re-install Sonoma I get the error ’The operation couldn’t be completed. (com.apple.BuildInfo.preflight.error 21)’.

IIRC, I've seen a few other posts recently which have mentioned this or a very similar error message. Unfortunately no one else has ever provided such good details about what led up to that issue. I thought it very strange to see such an issue.

If I can find some time and a T2 laptop, I may try dual booting a T2 laptop with Sonoma & Sequoia to see if I can reproduce your issue. No promises though. You do have me curious which is a good motivator.

It maybe? possible that the iBridge Firmware in the T2 Secure Enclave was updated when booting from Sequoia

Thusly the current issue ?

I did see a user who had upGraded to Beta Tahoe and later decided to back track to Sequoia

In their case, the Touch ID functionality ceased to work as expected

Secure Enclave Boot ROM

The Secure Enclave includes a dedicated Secure Enclave Boot ROM. Like the Application Processor Boot ROM, the Secure Enclave Boot ROM is immutable code that establishes the hardware root of trust for the Secure Enclave.

On system startup, iBoot assigns a dedicated region of memory to the Secure Enclave. Before using the memory, the Secure Enclave Boot ROM initializes the Memory Protection Engine to provide cryptographic protection of the Secure Enclave protected memory.

Until I can find the exact models of iMac's that do qualify for macOS 26 Tahoe

The best we can provide is

macOS Tahoe is compatible with these devices.

As for the Secure Enclave

I think the link in previous posting gives a general overview

Sorry for the delay, but I could not find my email notification for this thread which I had marked so I could access it again easily. After much persistence, I was able to find it again (I could not remember the issue exactly which made searching very difficult).

I do have a laptop I can use for testing, but must now find an external drive.

However, I did have an "Aha" moment about where I had seen this issue occur. I've seen this issue occur on M-series Macs when I go to select a default Startup Disk in System Settings. When I do that, some times the M-series Mac will reboot with your exact error. I suspect the issue is due to a pending macOS update that has been staged in an APFS snapshot, or that pending staged snapshot has somehow been corrupted. Every time I've seen this issue occur, there seems to be a staged update snapshot. I have tried to revert to the existing snapshot (or even choose the staged snapshot), but with all the new security features, this appears impossible although I did get one Mac to boot again for a single instance (I wish I had checked & installed any minor OS update patches). It has been very frustrating & annoying since I was only configuring the default Startup Disk because I had erased the NVRAM settings and wanted the user to have the Mac boot immediately to their SSD instead of wasting time searching for bootable volumes.

I hope to better assess the system next time prior to configuring the default Startup Disk option in System Settings so I may get more information about the issue.

Maybe make sure that macOS is fully patched by checking for new Software Updates prior to changing the default Startup Disk.

Are you actually going into System Settings and reconfiguring the default Startup Disk? Or are you Option Booting to access the other boot drive? I would say the latter should get around the issue if it is the same one I've seen with the M-series Macs. If the the other OS is "corrupted" when using the Option Boot screen, then something else must be going on (or most likely something else).

I'm checking out another possible option which could bypass the need to reconfigure the default Startup Disk with System Settings.

I have never seen such error message when doing multi-boot between MANY macOS versions on Intel Mac mini 2018 with T2 chip.

I always Option-boot so I get this selection for bootable test drives (OWC Envoy Pro mini which has bootable volumes for all macOS versions my Mac mini 2018 supports):

A minor issue is that Mojave has the following alert when it obviously does not yet know how to handle separate System and Data volumes or the "Update" volume (obviously the System snapshot?) in Catalina and later volumes:

So Mojave Desktop looks like this:

Catalina mounts only its own and Mojave Data volumes as well as those "Update" volumes and fails to mount later macOS volumes:

Catalina Disk Utility has the following error message when trying to mount those missing later macOS volumes:

An alternative, if you are open to the idea ?

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

2019 iMac & 2917 iMac Pro, no problems switching any OSes they’ll run including the 2 you mention.

Have you repaired the Sonoma Disk from Sequoia yet?

👍🏽