This array is a RAID 1 and in this case 1 of the 2 drives failed (a WD drive and I've found them to be the weakest and most unreliable of any brand and are easily damaged/DOA when shipping them).
mdadm --manage /dev/md0 --add /dev/sdb1
The above assumes the array you want to add to is /dev/md0 and the device we are adding is /dev/sdb1
*One thing to remember is to make sure the partition you are adding is the correct size for the array. You can also g........