Do not take the primary disk offline in a mirrored ZFS root configuration. The system will not boot from a disk taken offline in mirrored root-pool configuration.
Workaround: To detach a mirrored root disk for replacement or take it offline, boot from another mirrored disk in the pool. Choose one of the following methods:
Bring the primary disk in the mirrored ZFS root pool back online. For example:
# zpool online rpool c0t1d0s0
If the primary disk has failed or needs to be replaced, boot from another disk in the pool.