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 get away with creating the partition and running "partprobe" instead of restarting if the HDD is not in use already.
After that you can check /proc/mdstat and see the rebuild progress. It will be faster or slow depending on the current IO on the existing array.
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