The Pegasus R4i is designed to work with the unique design of the Mac pro to receive proper cooling. The airflow is more than adequate as there has been no issues with temperatures. However; there are some cases where the last two drives, usually PDs 2 and 4, will have higher temperatures than PDs 1 and 3. These drives are located closest to the front grille fans of the Mac pro.
By design the Mac pro will speed up the fans if the ambient temperature starts to creep up. If you'd like to manually speed up the fan speed there are 3rd party fans that can help you modify the fan speed. You can try crystalidea fan control.
https://www.crystalidea.com/macs-fan-control
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