For me this results eveytime. I believe is because UDMA CRC error count this happens because SATA connection or power supply to disk is lost.
Maybe a better power supply is an option.
Do a NAS power off and power on again
First of all, guarantee Parity is OK.
1. Stop Array
2. Remove disk from array
3. Mount disk in Unassigned Devices and check if all data is ok
If OK proceed
1. Next goto Tools -> New Config
2. Preserve current assignments: ALL
3. Apply
Next Start Array with parity is Valid checked.