As this forum post said. https://forums.freebsd.org/threads/56622/ I have a external USB raidz2 zpool, When it attached to the PC, zfs loader auto reboot when loading kernel. Remove it I can boot the system, When FreeBSD booted I can successfully import that zpool without any error message. If I use UFS root on the same PC, I can boot with the external USB zpool attached.
This error is dependent on the Mobo. I installed that internal disk and external enclosure to another PC, It works fine. Maybe there's a bug in the bios or zfsloader. I think it is hard to reproduce on other environment. If anyone require detail information, I will test it on my environment.
^Triage: I'm sorry that this PR did not get addressed in a timely fashion. By now, the version that it was created against is long out of support. As well, many newer versions of ZFS have been imported. Please re-open if it is still a problem on a supported version.