# first i forced the array to start in degraded mode
root@nfs:~# mdadm --assemble -s -f
# then i had it add the drive back into the array
root@nfs:~# mdadm --manage /dev/md0 -a /dev/sdd1
# then i go on vacation for a week while this completes...
root@nfs:~# watch cat /proc/mdstat
That's it. Hopefully it won't happen again.