Created volumes and partition not showing up in Startup Disk

Following this because as AFPS evolves it is getting more difficult to have dual boot partitions on a single physical drive. It is occasionally very handy to be able to easily switch from OS version on a single machine.

Hi jamie_cd,

We understand that you're having trouble creating a bootable installer. We'd be happy to help you out with this. To clarify, are you connecting an external hard drive or are you attempting to do this on your internal hard drive?

To create a bootable installer, please follow these steps: How to create a bootable installer for macOS

Should you run into any issues doing so, start up in safe mode to see if anything changes: How to use safe mode on your Mac

If needed, try creating one from another user account. If you don't have one, you can create on in the Apple menu > System Preferences > Users & Groups.

I successfully CCC cloned Big Sur and installed Mojave on an external disk's two APFS volumes. I usually Option-boot and then do the selection but they do show also in Startup Disk pref when I want a more permanent setting.

How did you Install Catalina to the other volume?

"Some Big Sur startup volumes don't appear in the Startup Disk Preference Pane"

https://bombich.com/kb/ccc5/macos-big-sur-known-issues