*NORCO DRIVE TRAYS AND PROPER AIRFLOW*
If you own a Norco RPC-4220 or RPC-4224 server case, you may have noticed that the drive trays contain an ingenious slide mechanism to open or close the the front air vent, for the purpose of decreasing airflow to drive bays that are empty. After a bit of research and contacting Norco I confirmed that while these cases generally ship with trays defaulted *open*, at some points they have shipped with drive trays defaulted *closed*. Since these cases don't ship with any instructions, the danger is some people not noticing the slide mechanism, and mounting their drives into trays with closed vents - I did.
Check your vents! Any trays with drives, open it. Any trays without drives, close it.