Unable to install 15.7 on external SSD: Error 108

[Joymaker]

Some months ago I bought a SanDisk external SSD and installed Mac OS 15.5 on it. No problem, booted fine, has booted ever since.

Now I need 15.6 or better for the newest XCode. I tried updating to 15.7 using software update. Result: it goes through the whole update process, taking about an hour, and then at the final reboot I find myself back at 15.5 as if nothing had ever happened. So I tried explicitly downloading the installer and doing it that way, exactly the same result.

Next plan: I created a new clean partition (hard partition, not APFS volume) and installed 15.7.5 to it. After the install completes, I find myself back at my primary OS, not in 15.7.5. And when I go to Startup Disk and select it, it selects, it shows 15.7.5, but then at the last minute when I tell it to reboot to the new disk I get:

Unable to set startup disk
An error occurred while setting “NewOS” as the startup disk: The operation couldn’t be completed. (SDErrorDomain error 108.)

Curious what experience others have had with this. Is it really true, as I read in one comment, that you have to create a brand-new username rather than letting your present user from the internal drive "own" the new volume? Even though it invites you to use the existing user?

Or is there some new limitation on external booting that was introduced between versions 15.5 and 15.7?

Using a 2021 Macbook Pro with Apple Silicon M1 Max processor.

 

[Fishrrman]

Just wondering, you may have already considered this, but...

... were you using the USB port that is also used for DFU tasks when you tried to do the upgrade?

From what I've read, that won't work. You must use one of the "other two" ports on the MBP to create (perhaps "update", also?) a bootable external drive.

I also have a 2021 MBP, and I believe the DFU port is the one one the left side (of the two) that is CLOSEST TO the display. So... you would want to use the USB port on the left that is closer to YOU, OR use the one on the right side.

Again, this may or may not be a factor.

One other thought:
You might try to erase the entire external drive, and start over from scratch.
You may have done this already, too.

 

[Brian33]

As @Fishrrman says above, try a different Thunderbolt port.

According to eclecticlight "Apple silicon Macs have 2 types of Thunderbolt ports", you can't install macOS or update macOS on an external drive when it's connected to the (one) designated DFU port. After install/update, it apparently will run fine connected to any of the thunderbolt ports.

Hence, every Apple silicon Mac has one Thunderbolt port designated as its DFU port. That’s used to connect it to another Mac when in DFU mode, but can’t be used to install or update macOS on a bootable external disk.

To identify: https://support.apple.com/en-us/120694
BUT... I remember seeing something that claimed there was an error in it...
https://lapcatsoftware.com/articles/2026/2/1.html