/dev/sdb4 was a part of /dev/md2, but was removed manually. After reboot: # cat /proc/mdstat Personalities : [raid1] md2 : active raid1 sda4[1] 404468416 blocks [2/1] [_U] # mdadm /dev/md2 -a /dev/sdb4 mdadm: /dev/sdb4 reports being an active member for /dev/md2, but a --re-add fails. mdadm: not performing --add as that would convert /dev/sdb4 in to a spare. mdadm: To make this a spare, use "mdadm --zero-superblock /dev/sdb4" first. How do I readd sdb4 to md2?
Just follow the instructions provided in the output from mdadm. The issue with raid1 is that if you just re-add it, mdadm wouldn't know which of the two drives is authoritative and you could end up with old data. If you mdadm --zero-superblock /dev/sdb4 first you should be able to re-add the drive.