ZFS failed to read pad2 area of primary vdev

There are number of issues that can lead to this error when trying to boot a FreeBSD server with ZFS as root / zroot:

ZFS failed to read pad2 area of primary vdev

There can be a few reasons for this error, but check these in order:

• “Phantom” Pool: Make sure you haven’t added any disks, thumb drives, etc that could have possibly been formatted and have the same ZFS pool name (ie; zroot) as your normal boot disks’ boot pool. This one is easy to miss especially when replacing a failed disk with a supposed blank disk.
• Old Boot Loader: Ensure your boot loader partition are on the latest copy from your OS. This can be done by booting using the appropriate live ISO or thumb image and then updating the MBR and/or GPT boot blocks.

  As an example for GPT partitioning this would update the boot code on disk ada0:

  gpart bootcode -p /boot/gptzfsboot -i 1 ada0

  IMPORTANT: check your devices and partitions before running!
  Make sure you update any other disks if you are running a mirror, too.