In my "case", I don't think adding any extra fans will make any difference. It already has 5 internal fans, not counting the one on the CPU or video card. I am using the Antec P182 which is the same as this:
Except I got the gunmetal one and not the one with the stainless steel outsides. A flash demo of the cooling paths can be seen here:
We are getting a heat wave this week and it started yesterday. For us, anyways, 90 degrees F is like the rare upper temperature limit we ever experience. And having a lake nearby, we also get some high humidity. Yesterday we had about 90% humidity. I would imagine that humidity effects computers as well?
And no, there is no air conditioning! Maybe someday I can get it... we'll have to see how much it costs.
Adding fans for cooler temps ... maybe, may not.
Fortunately it is a simple thing to test! Open the case, run the system, note the temps, now just rig a spare fan ( to an old PS if necessary ) and hold it so it blows air directly over and under the HDD and watch the temps. NOTE: as obvious as this sounds, beware of the spinning fan blades with both your fingers and wires in the case!
For real hot things like chips you get several variables you can play around with: thermal paste, heat sink, fan. But for packaged components like a disk drive you only got one, air flow. Convection depends on moving air to transfer heat from a heat sink ( which in the case of a HDD is the entire surface area ). And it really requires just a little air movement to work well, indeed you'll hit diminishing returns eventually with more and more fans, all the gain is on the low end of the curve, moving from no air-flow to some air-flow is a big jump.
So when you say adding fans might not help you might be right, but most likely there is not continuous good airflow over both sides of the HDD package, the metal top and the PCB bottom. Or perhaps an unusual circumstance where warm air is being blown right over the HDD, it can happen from a ribbon cable having moved and diverting air movement.
A 120mm fan can perfectly "cool" ( well, increase convection on ) three internal HDD's, and a fourth can be fitted in a pinch.
BTW, we are also in a heatwave, 90 degrees F with about 100% humidity for nearly 3 weeks now. No HDD or any other temp problems though.