Unable to Boot Sonoma using an external SSD on Mac

I got this issue, but it was resolved by changing the external enclosure to Thunderbolt 4, but I got another issue which keeps auto restart after period of time.

I had a similar issue using Carbon Copy Cloner and the legacy feature to make it bootable. It will not boot so I had to start in recovery mode and then install Sonoma on the eternal disk then it works fine. Never had that issue in prior version so something changed with Sonoma.

FWIW, I cloned my Sonoma install running on an iMac Pro to a SanDisk Extreme Portable SSD using SuperDuper!. No problems booting from it.

vabmotto wrote:

I've been using an external SSD to boot Mac OS X for years. I have a Mac Studio and a MacBookPro (M1), and I was successfully booting both from an external SSD on Sonoma 14.1.1. However, when I go to upgrade to either to 14.2 or 14.2.1, neither the Mac Studio, nor the MacBookPro will upgrade. They both look like they are going through the upgrade successfully (they download 14.2.1, reboot, it sits there for a while looking like its upgrading ... but when the system finally comes back up and I login, it is still running 14.1.1. I then tried to create a new bootable SSD, and it all looks like its going fine too, until its time to reboot, and that is when I see the same error as was mentioned.

Error:

Unable to set startup disk. An error occurred while setting "Machintosh SSD" as the startup disk: The operation couldn't be completed. (SDErrorDomain error 108.)

You really should start your own thread since your issues may not be entirely the same as the OP's. Yes, they may be similar, but your system configuration and method of making the external boot drives may be completely different so may require completely different suggestions.

For example:

Was this external drive used exclusively with just one Mac or multiple Macs? This has become very important for external macOS boot drives on Apple Silicon Macs because there is a new concept of "Ownership", so I'm not sure if you can share macOS boot drives between different Apple Silicon Macs. You definitely cannot share a macOS boot drive between an Apple Silicon Mac and an Intel Mac since it would require reinstalling macOS.

Was macOS originally installed to this external drive or did you create a bootable clone? Bootable clones of Apple Silicon Macs are iffy at best....some people seem to have no problems while others have issues.

Is Filevault enabled on the Mac? Was it enabled on the external boot drive?

I've been using an external SSD to boot Mac OS X for years. I have a Mac Studio and a MacBookPro (M1), and I was successfully booting both from an external SSD on Sonoma 14.1.1. However, when I go to upgrade to either to 14.2 or 14.2.1, neither the Mac Studio, nor the MacBookPro will upgrade. They both look like they are going through the upgrade successfully (they download 14.2.1, reboot, it sits there for a while looking like its upgrading ... but when the system finally comes back up and I login, it is still running 14.1.1. I then tried to create a new bootable SSD, and it all looks like its going fine too, until its time to reboot, and that is when I see the same error as was mentioned.

Error:

Unable to set startup disk. An error occurred while setting "Machintosh SSD" as the startup disk: The operation couldn't be completed. (SDErrorDomain error 108.)

Same here. Finally gave up trying to get around the ridiculous insistence that cloud drives must be on the system disc (not so sneaky attempt to upsell everyone to a 1TB+ drive maybe 🧐) and installed from fresh on a 2TB WD external SSD.

The odd thing is that it’s been working fine for a whole week, restarting and rebooting fine. This morning I shut down completely and now it won’t reboot??

I can see the disc in recovery mode. I can run checks on it with disc utility. I can change boot security options and I can specify it as the startup disc.

It just won’t boot. I hear the chime and the monitor wakes, but then goes to sleep. The power light on the Mac is on, but it never boots 😭

sonoma 14.2.1 installed last week (when the Mac was still behaving).

What version of Sonoma?

I am having your issue. Did you ever open up another ticket or get it fixed? Your response is much appreciated!