I find the ventilation holes are quite tiny, even though they look like slits on the outside, these slits are blocked on the inside of the case to leave only a small pair of holes, about 1 mm in diameter. This leads to very low airflow which is not a very good design for a device that calls itself a "server". There is a very noticeable difference in the fan speed when the case is opened.
I have a pair of Hitachi HDT72110SLA360 1TB drives and they get quite warm to the touch. I think it is more than 45C when they are running.
There is no spin-down or sleep mode while idle. I have another NAS that has a timeout after no activity, it will put the drive in a lower power mode and this reduces the temperature of the hard drive to what feels like room temperature.