I don’t know why I always forget this command, or why it is so hard to discern from the man page / –help information. I just had my drives in an existing RAID array get out of sync and needed to rebuild the one that was down. This did the trick:
mdadm /dev/md0 -a /dev/sdb1
This is on a software RAID array called /dev/md0 made up of /dev/sda1 and /dev/sdb1.