mdadm: power failure during raid1 rebuild
Solution 1:
Both your drives appear marked as "spare". Stop the array then try reassembling it with the scan option:
mdadm --stop /dev/md0
mdadm --assemble --scan
If that doesn't work, try
mdadm --stop /dev/md0
mdadm --assemble --scan --force
Check that everything is back on line. Also please report what /proc/mdstat looks afterwards.
If you want to be on the safe side, disconnect one of the drives beforehand, and start the array with only one drive. There are only 10 events of difference between the two drives, so they're probably both OK but for a quick fsck.