Catalina Installation «fails» on external SSD
I've bought a Samsung T5 500GB to give back some speed to my late 2013 iMac. I erased the SDD (Journaled, GUID), downloaded the installer for Catalina. The installer starts but when it reaches the point to restart the Machine to install Catalina on the external Drive it restarts, but boots right back up from the internal drive. No OS installed on the SSD.
What am i doing wrong?
iMac 27″, macOS 10.15