I was caught flat-footed when trying to import a used disk into a mirror set. One of the disks in the mirror set was removed for disaster recovery trials.
Inserting the disk back did not start a re-build. Not surprisingly, Dell PERC marked it as foreign. One disk is shown as online and another rebuild. Going by the LEDs, the mirror set was not rebuilding. I know I need to do something, but what?
After some Internet searching, I need to clear the foreign configuration on the used/rebuild disk. Thereafter flag it as a Global HotSpare.
Voila! rebuild starts!!!