Yep, HEAT, or more heat from being in an enclosed cabinet or other furniture for TV/audio equip can greatly accelerate the death of an HDD. The solution I and many others on this forum have found if we have to have an STB in an enclosure is to use a laptop fan and place the STB upon it. It is quite effective with greatly reduced temperature noticed from the display of the "Tests" list. Yes good air flow no matter where you place your STB.
Also, the same problem if the STB is placed on top of or even under another device that creates it own high heat. I ALWAYS place Dish DVR's upon clear plastic casters (or sometimes krazy clue 2 or 3 casters for added height if atop another heat producing device) to allow for airflow UNDERNEATH the box, as well. I've observed that airflow UNDER the Dish STB seems to be very important.
Now, it isn't so critical to use casters with TiVo because the fans on TiVo are always running and running pretty fast such that you can't help but hear them working. So, TiVo units--except the new base Romio unit) is doing a much better job of cooling because the fan is really doing the work it should. However, while the Plus and Pro models of Roamio continue to cool this way, the base Roamio does not cool as efficiently do to it's internal design. I've had people complain that the nightly reboot of the ViP Dish DVR in their bedroom would wake them up at night. NOT ME.
The reason Dish does not have the fan activate except in conditions when the STB is getting too hot is that Dish has gotten LOTS of complaints over the many years from customers who say the sound of the fan is bothersome, especially in bedrooms, but people complain even if the STB is in the living room. Does anyone remember the BIG complaints about the 721 fan being on all the time and it bothering people. The fan really needed to be on, but eventually Dish fixed it so the fan was OFF except for extreme conditions. Dish probably fixed it so the 721 wasn't doing too many things at the same time as a way of reducing the heat and, thereby, being able to shut the fan off.
The sound of a fan working at moderate levels just does NOT bother me. Of course, when the fan is really working at high level, it sure can be annoying, but, again, at moderate or normal level it does not bother me in the least even though I can hear it, but there sure are a lot of people who can't tolerate the fan at a normal level. I suppose we are all different.