Unlike Intel Macs, M1 series Macs can’t enter Startup Manager, to pick a boot volume early in the boot process, using the Option key. There are two main methods available on M1 models: the Startup Disk pane in macOS, and entering Recovery, where a different Startup Manager can be accessed either from the opening screen or later in the Apple menu.
M1 models also require that all boot volumes have an owner, and ownership has been one of the more troublesome features of external boot disks. Prior to macOS 11.4, those problems were often so severe as to prevent booting from an external disk altogether. Although this has come a long way since, it still isn’t without the occasional glitch.
In macOS 12.0.1, I’ve experienced problems getting ownership to work with an external SSD containing multiple boot systems, which had previously worked smoothly. When I selected one of the boot volumes in Recovery’s Startup Manager, it warned me that the volume I had chosen had no authorised users. This problem usually reflects an ownership issue, and should be resolved by authorising the current user for the boot volume.
When I did this, the dialog confirmed success by placing a green tick against the newly authorised user, but continued to refuse to boot from the external disk, reporting yet again that the volume had no authorised users.
The workaround is to select the internal SSD as the boot volume, restart from that, and there use the Startup Disk pane to change the boot volume. While that takes you through the same process of authorising a user for that boot disk, in macOS this was successful, and didn’t cause an endless loop.
This has been eased in Monterey 12.1, but there are still occasions when there’s a dance of boot volumes, for which this remains the best solution.