macos sonoma download

Attempting to reinstall macOS sonoma on external drive ... following m3 chip instructions but after the reboot, the mac boots into internal HD, not the external drive selected so the install never completes.

have reformatted external drive several times and have been using this drive for the beta testing for several years --

after several failures using the recovery option, am attempting to download from app store the macOS sonoma dog, but it is failing to complete downloading with an error (now can't remember what the error was).

what am i doing wrong?