There is no indication or user interface option to determine if/when disk mirroring has occurred on the ProxySG appliance.
If you have received a new disk and have already inserted the new disk into the appliance, the disk mirroring processes have already begun.
There are no indicators or statistics to view or verify when or how mirroring is taking place.
Blue Coat suggests leave the disk in place for a day to make sure all configuration and settings have been replicated. Simply monitor and verify that the replaced disk is detected and health check is nominal for that disk.