RN312 says volume is degraded

I have a ReadyNAS 312 that has 2 4TB drives in RAID1.  Disk tests complete with no issues and the logs show both disks are online.  volume.log shows device md1 is a RAID 1 and has a redundant state.  It also shows device md0 is a RAID0 with a state of degraded with only the sdb drive connected. I don't think the drive has failed. What needs to be done so the volume is no longer degraded?  I'm happy to send logs if needed.