Unable to update external SSD from macOS 15.7.3 to 15.7.4

Joy at last.

I did a recover, got to Disk Utility. I used Disk Utility to thoroughly First Aid the external SSD. I saw three levels of that SSD and I used Disk Utility separately on each one. The innermost layer had 77 packages to examine and that took hours. No problems reported. Then I went on and installed the Mac OS again. This time it updated the external SSD which now shows 15.7.4, the same as the internal SSD.

At the end, the computer booted to the external SSD.

I don't know which of the steps I took corrected the problem. I think the only thing I did differently was to use First Aid in Disk Utility to so thoroughly check the external SSD.

Until today, I was unable to update the system on the external SSD to 15.7.4

I outgrew my internal 1 Tb SSD. I bought and installed an external 2 Tb SSD and installed Mac OS on it. The computer has been booting from the external SSD for probably over a year. It was updated to the current version Mac OS when new ones were available. I usually held off to the version xx.1 before the upgrade.

Occasionally after a power outage, the computer would boot to the internal SSD with whatever version Mac OS was there. I created a special background screen to alert me every time this happened and always used Startup Disk to reboot to the external SSD. This always worked, sometimes it took several reboots.

Yesterday, the computer booted to the internal SSD. After several tries to boot to the external SSD, I decided to update the internal SSD to the latest Mac OS available for this age computer. That is 16.7.4. Now I think this was a big mistake.

I have tried to use Startup Disk under General and I have tried to use Command R to choose the external SSD. I am unable to boot to the external SSD.

I did a backup of all the data files on the external SSD to and external HD just in case I have to erase the external SSD and reinstall the Mac OS but I am unsure this will solve my problem.

I have had three chats with Apple support trying to resolve the problem. No joy.

Camelot,

My reply at 2:41 PM explains what the problem was and how I fixed it. I'm not sure I've solved it but for the moment I've fixed it. I am now booted from the external SSD which has Mac OS 15.7.4 installed.