First of all, I have 'Auto-Rebuild' disabled.
When the unit shows 'DEGRADED' again, login onto the config page.
Goto the 'STATUS' tab. Under drive info, you would see a list of drive with the drive serial number and the drive status displayed. Just copy down the serial number with the status 'DEGRADE'
Then proceed to rebuild the volume. When it degrade again, check again to see if the same drive is degraded.
The tricky part is if you have more than one drive in distress, diagnosis may be a little tricky. But the procedure is still the same.
Have fun!