That seems like a very poor chassis design on the part of the NVR manufacturer. The average modern 3.5" high capacity HDD doesn't generate that much heat. Even

That seems like a very poor chassis design on the part of the NVR manufacturer. The average modern 3.5" high capacity HDD doesn't generate that much heat. Even 'datacenter' HGST drives average around 5.5W and will top out at 7.8W TDP under maximum stress. Designing a case that uses relatively low rpm, quiet, 120 or 140mm 12VDC fans to pull air through it and cool six or eight hard drives isn't that difficult. In a midtower desktop PC case set up as a NAS with a low wattage CPU, used as a NAS, a single 140mm fan at the rear sucking air from front-to-back is often quite enough to cool eight 3.5" HDD. But equipment designers keep trying to stuff things into spaces that are too small and use inadequate ventilation.