GEOM: the secondary GPT table is corrupt

[Barnard, Jeremy]

Do you have any idea how to clear up:

GEOM: mirror/gm0: the secondary GPT table is corrupt or invalid.
GEOM: mirror/gm0: using the primary only -- recovery suggested.

It has not caused any issues, but I'd still like to eliminate the errors without causing data loss.  I have two WD drives in a software mirror booting freenas from a USB stick.