So, if I understand you correctly, you are on a iMac and want to boot from a bootable USB-Key. Have you tried the following:
`Startup Manager
The Startup Manager allows you to choose which bootable volume to start up from if you have more than one connected to your Mac. You can also use this method to startup up from a Time Machine backup if it’s connected to your Mac.
For an Intel-based Mac:
Hold down the Option (Alt) key while you start up your Mac.
Choose the volume you want to boot from by clicking on it when the options appear on the screen.
Press Return.
For a Mac with Apple silicon:
Go to Apple menu > Shut Down.
Press and hold the power button, and do not release it until the “Loading startup options” message appears.`
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