So: mdadm --add /dev/md0 /dev/sdb succeeds. However, mdadm --detail /dev/md0 does not show sdb after I run that command. As best I understand, /dev/md0 is a container for /dev/md126, which is SUPPOSED to be a RAID 1 mirror between /dev/sda and /dev/sdb.
I have a feeling I'm doing it wrong, but I'm not sure how to actually fix my degraded mirror (which is bootable since /dev/sda is fine)
/dev/sdb is a blank drive.