Re: FreeBSD boot from root pool larger than 1Tb/cannot read MOS/all block copies unavailable

From: Charles Sprickman <>
Date: Wed, 27 Apr 2022 00:10:46 UTC
> On Apr 26, 2022, at 10:10 AM, Eugene M. Zheganin <> wrote:
> Hello,
> recently I found another server that was running 12.x and became unbootable because of the famous "cannot read MOS/all block copies unavailable/etc" gptzfsboot message which can be randomly displayed when boot blocks migrate beyond 1st Tb on a large root pool.

Is this documented somewhere? I’m pretty sure I have a number of servers that fit this description. How do boot blocks “migrate”?