Last week, one of my FreeBSD 11.2 servers failed to boot with the following error after freebsd-update. Mounting from zfs:zroot/ROOT/default failed with error 6; retrying for 3 more seconds It was the first time I encountered this type of error. After struggling for some hours, I was able to boot the server by disconnecting an external USB harddrive, which I had prepared as a secondary backup storage. But since then, I had been wondering what the root cause was.