Repairing a degraded raid mirror. Assuming sdb1 has fallen out of the mirror:
Check to see if the disk thinks it should be in the array
mdadm -E /dev/md0 mdadm -E /dev/sdb1 mdadm -E /dev/sda1
Readd the missing disk
mdadm /dev/md0 -a /dev/sdb1
Check to see the array is rebuilding
mdadm -D /dev/md0