I'm trying to get 10.15 installed on an external SSD. I started by using recovery mode to install Mojave on it from a MacBook Pro. That all worked fine. I then booted into the new installation and downloaded the beta. The installer runs for a while and gets to the point where it asks to restart. When I click the restart button, some apps close, but then it says "An error occurred while preparing the installation." I can't find any logs or details on why exactly it's failing.
Has anyone gotten the beta working on an external drive?