Can't install macOS Monterey beta on M1 external drive

Same here, getting: "An error occurred while setting “MacOS 12” as the startup disk: The operation couldn’t be completed. (SDErrorDomain error 108.)" when trying to boot to external drive on M1

Just tried to install on Intel MacBook external drive. Like expectedly it works.

My first attempt was like everyone else. A spare partition of 256GB on thunderbolt drive. So I had good backups and decided to install on Internal m1 Mac mini disk. This erased the installer so in terminal I:

softwareupdate  --fetch-full-installer

which downloaded the installer. The first time I ran it, it errored out. So I ran it again and it installed and boot the new part. This makes me wonder about the deterministic nature of this. So if you run Monterey, you can get an external disk running it.

What finally worked for me was installing the macOS 12 beta from inside the recovery mode:

1. Download the Monterey installer app onto your internal SSD.
2. Boot your M1 Mac into recovery mode by shutting it down first and pressing + holding the Touch ID key.
3. Connect your external drive and format it using Disk Utility; choose APFS as file system.
4. Disk Utility can also be used to unlock your Mac's internal SSD in case it's File Vault 2 encrypted.
5. Still within the recovery mode, open Terminal, navigate to the Monterey installer location (probably ~/Downloads on your internal disk)
6. Start the installation process by executing ./Install macOS 12 Beta.app/Contents/MacOS/InstallAssistant

Note that the installation may take significantly longer, especially towards the end.

In my case, I tried to install Monterey beta into a volume created in the Big Sur's APFS conainer. https://gyazo.com/2f0275ed0d7b4f120a4d8cce8a854e99

At the time of restarting, it failed (waiting more than few miniutes after quiting visible apps). So I restarted myself. After restarting, Big Sur is booted. I tried to select the Monterey volume as the start up disk (System Preferences -> Startup Disk).

I got an error:

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

https://gyazo.com/427104df6897d5adc65d3222c2c95dfb

The key (at least for me) was to use a Thunderbolt 3 SSD. After various attempts with a USB-C Samsung T5 I came across some articles (to which I can't like on these forums for some reason... - search for "How to boot an Apple Silicon Mac from an external drive") that pointed out that requirement.

I then ordered a Samsung X5 external Thunderbolt 3 SSD and the installation went and worked fine with that.

For me (on my M1 Mac mini) the solution was to connect my Samsung T5 SSD via a USB-A-port and not via USB-C. After that the installation could be done without any problems.

i solved it first install big sur on external drive and then upgrade it to moterey. used t7 2tb.

I used SanDisk 1T with Apple thunderbolt cable ( important) installed it again today since my Mac Studio wouldn’t let me make external SSD boot Monterey so I made one on M1 Mac Mini and use it on Mac Studio