We received a client machine a while back. At some point they needed more disk space and sent us a pair of drives with instructions for mirroring data from the existing machine to the new drives.
When I looked at the drive I noticed something odd:
md4 : active raid1 sdb8(S) sda8 1942145856 blocks super 1.2 [1/1] [U]
While this was a Raid 1 set, notice that the indicator shows that sdb8 is a Spare (S) and that the Raid 1 set has 1 of 1 drive and isn’t broken. This usually occurs when someone is doing an in-place migration to a larger drive and creates a Raid 1 partition with a single device and forces it online with the intention of later adding the second drive. Had the primary drive failed, they would have experienced data loss.
To fix it:
mdadm --remove /dev/md4 /dev/sdb8 mdadm --grow /dev/md4 --raid-devices=2 --force mdadm --add /dev/md4 /dev/sdb8
and we see the resulting md status:
md4 : active raid1 sda8 1942145856 blocks super 1.2 [2/1] [U_]
After the reconstruction we now have a properly configured Raid 1 set.