If a disk in a mirrored root pool fails, you can either replace the disk or attach a replacement disk and then detach the failed disk. The basic steps are like this:
Identify the disk to be replaced by using the zpool status command.
You can do a live disk replacement if the system supports hot-plugging. On some systems, you might need to offline and unconfigure the failed disk first. For example:
# zpool offline rpool c1t0d0s0
# cfgadm -c unconfigure c1::dsk/c1t0d0
Physically replace the disk.
Reconfigure the disk. This step might not be necessary on some systems.
# cfgadm -c configure c1::dsk/c1t0d0
Confirm that the replacement disk has an SMI label and a slice 0 to match the existing root pool configuration.
Let ZFS know that the disk is replaced.
# zpool replace rpool c1t0d0s0
Bring the disk online.
# zpool online rpool c1t0d0s0
Install the bootblocks after the disk is resilvered.
Confirm that the replacement disk is bootable by booting the system from the replacement disk.