mountroot prompt with error2, when trying to boot from a single drive in a 2-way mirror

[yudi v]

Hi all,

I was testing recovery scenario by removing one of the drives in a 2-way
mirror, but the system fails to boot and comes up with the mountroot prompt
with error 2. When I reconnect the second drive, it boots fine again.

Any suggestions on what the problem might be?

it's a simple root-on-ZFS setup (9.1 upgraded to 10.1 recently) with two
disks in mirror config.
each disk has 3 partitions, first one has the boot code, second has the
swap, third has the OS.

and the zfs pool is setup on the 3rd partition of the two disks.

-- 
Kind regards,
Yudi