The results seem to counter our general recommendation to give hard drives plenty of ventilation room in “The Care and Feeding of External Hard Drives” (28 April 2014). Overall, Backblaze found no correlation between heat and failure rates, with one exception: the Seagate Barracuda 1.5 TB drives, which failed slightly more often when run at higher operating temperatures. But the Hitachi drives surveyed actually failed a bit more frequently at cooler temperatures. It’s worth noting that the drives experienced average temperatures between 22° and 30° Celsius, well below the 55° to 60° maximum operating temperature recommended by drive manufacturers.
In any case, it’s still important to run drives within their specified operating temperatures, which means giving them sufficient ventilation. As long as you don’t block cooling vents or put them in insanely hot environments, they should not be adversely affected.