K447,

My server case has two main sections. The section above for Mother Board, Video Card, CPU, expansion cards, etc, is extremely well cooled. Two 12cm Noctua Case Fans, plus 1 14cm CPU heatsink fan, produce what i consider an ideal airflow.

Underneath, case section devoted to PSU, Optic unit, HDD, is instead poorly cooled. There is no fan other than the one mentioned in this thread. ALso, there is not much space, and several cables (fromt he PSU to the above section and to two SSDs, 2 HDDs, and DVD reader), and SATA cables to all the same units from the above motherboard.

So, yes, you are correct, air flow is actually significantly compromised by all cabling. I was thinking precisely to see if I can re-route them in some more orderly way to improve the situation.

I do not have a thermometer in my studio, but I'd guess the room is around 25°C . Delta between environment and HDDs is therefore between 5°C and 10 °C

Edit:
Bottom section has 2 locations for 2 8cm fans. Unfortunately, they'd be at the opposite side of the HDDs, separated from them by a lot of cables, which is why I decided to go with a larger fan close to the HDDs (see pictures above) and related case modding that I've been mentioning in this thread.