Trying to Install Big Sur on an External SSD While Running Monterey on Main Drive

[fsa526]

I have Monterey (12.3.1) running on my Mac mini (M1, 2020). I have an externally connected SSD drive on which I have installed Big Sur (to use programs that are not compatible with Monterey). The install went fine, and the computer restarted. I shut down and entered into recovery mode to choose the external drive with Big Sur. Unfortunately, when I click the drive, nothing happens (attached files). How do I get this to work?

Edit: I tried to upload a video of what happens, but it did not work. It was a video of me clicking on the icon for macOS Big Sur with no reaction from the system (just a brief spinning/loading wheel below the icon).

 

[Fishrrman]

You probably already did this, but...

... have you booted to recovery, then opened Startup Security, and DISABLED security...?
You have to do that before the Mac will boot from an external drive.

 

[mr_roboto]

You probably already did this, but...

... have you booted to recovery, then opened Startup Security, and DISABLED security...?
You have to do that before the Mac will boot from an external drive.

What? No, you do not have to reduce or disable security to boot from an external.

 

[Fishrrman]

roboto posted:
"No, you do not have to reduce or disable security to boot from an external."

Here's a screenshot of Startup Security (this one running on a 2018 Mac Mini):

 

[bradbomb]

What? No, you do not have to reduce or disable security to boot from an external.

Yes, you do when it involves booting a version of macOS older than what's on the internal SSD. Since the main drive is on Monterey, you need to reduce security to allow an older version to be booted.

How to boot an M1 Mac into an older version of Big Sur

How changing a volume’s boot policy can let you boot in older macOS, and update, if you wish. For anyone who uses external bootable disks with an M1 Mac.

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