Boot iMac from external SSD

when i set up an external startup drive for big sur, i didn't use a bootable clone. i did a clean install of the macOS on the external drive and then manually moved the things i wanted onto the external drive. to set it up, i started out by using the instructions in the well written user tip supplied below.

How to Setup and Use an External SSD as your Startup Disk - User Tip

now, when i want to boot into the external, i hold the option key on startup until i see the screen where i choose which drive i want. or if you are going to exclusively use the external drive, set it as your startup disk in SystemPreferences > StartupDisk

Once you erase the entire disk and format it as the following:

you'll need to setup CCC as shown, Control (right) - click on the destination drive and select "Legacy Bootable Backup Assistant…" from the contextual menu.

Then click Start in the next window. This takes less than about ⅓ the time it takes to do a fresh install and migrate from the original boot drive or Time Machine with Migration Assistant.

When you make a clone using CCC (V5.1.27) it asks if you want it to be bootable (which can easily be over looked). If you missed this step then it will not work. I have made many CCC bootable drives and all work fine.

The latest version, 6.0.2 also has the Legacy Bootable Backup Assistant…

Here's a screenshot of it:

May want to head over to CCC site specifically here >>> https://bombich.com/blog <<< specifically this area Does this mean that we can no longer have bootable backups?

If I were you and I am not - think the Method outlined by @ Jefferythefrog will work very well and achieve the desired outcome.

That or as CCC outlines their method

Old Toad, is this version 6+ of CCC? It looks different from mine.

No. I'm running Version 6.0.3-b2 (7185). Started using the beta version a while back and get updates to it. You can try to get the beta version at the developer's website.