If first disk of IMSM RAID1 is failed but still present in the system,
the array is not auto-assembled. Auto-assemble uses raid disk slot from
metadata to index disks. As it's not set, the valid disk is seen as a
replacement disk and its metadata is ignored. The problem is not
observed for other RAID levels as they have more than 2 disks -
replacement disks are only stored under uneven indexes so third disk
metadata is used in such scenario.