I attempted to update from TrueNAS 12U1 and the update failed. I waited an hour and attempted again. The Update completed with no issues that I can tell.
I am getting an error alert now:
System core files were found in /var/db/system/cores. Please create a ticket at https://jira.ixsystems.com/ and remove these files.
2021-02-09 18:02:49 (America/New_York)