I have LaCie 2big thunderbolt 2-disk device configured as a RAID1 from Disk Utility. One of the drives appears as "failed", so I unmounted the raid, replaced the drive and plugged it all back in.
Now Disk utility reports that one of the slices is missing from the RAID, and it shows my new disk separately on the list to the left.
The "slices", including the missing one, have a lock icon in front of them, and even after dragging my new disk to the list I can't rebuild the raid.
How do I rebuild the raid?