ZFS: i/o error - all block copies unavailable on reboot

I am trying to create RAID-Z pool on 3 disks, but I'm getting this error after reboot:

zpool create -f -o altroot=/mnt datapool raidz vtbd0p3 /dev/da0 /dev/da1

Error is

ZFS: i/o error - all block copies unavailable
invalid format

FreeBSD/x86 boot
ZFS: i/o error - all block copies unavailable
can't fild dataset /boot/kernel/kernel
Default: datapool/boot:/boot/kernel/kernel
boot:

It seems to me that you trashed you FreeBSD installation, creating a new zpool on /dev/da0 and /dev/da1 (overwriting what was previously there).

I strongly suggest you to never create a pool with -f unless you really know what you are doing.