A quick guide to mirror a ZFS root pool:
1. Attach the new device to your root pool:
# zpool attach
2. Install the bootblock on the new device:
# installgrub /boot/grub/stage1 /boot/grub/stage2
# installboot -F zfs /usr/platform/`uname -i`/lib/fs/zfs/bootblk
3. Make sure your OS of choice knows about this. On Sparc you need to add the device to the OBP variable “boot-device”, and on x86 to the BIOS settings.