Thanks Terence. Smartctl showed both disks to be ok so I ran sudo mdadm --manage /dev/md0 --add /dev/sda2 and it went ahead and added the the disk and rebuilt the array. All is looking to be ok again now.
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sda2 sdb3