also, just another point to consider about heat and hard drives. here is story involving my 2 seagate drives (i only have one now).
i used to watercool the non pcb side of the seagate drives, however, DTEMP gave an internal temp reading of never more than about 36C. so the drive casing itself was quite cool (cooler than when i was air cooling the hard drives). however, i got lots of errors. i would get messages saying that the drives had been removed and partitions couldnt be recognized. when i would reboot, the drives would sometimes not get recognized. in the end i was getting ready to RMA the drives (one at a time, cuz i couldnt be without both hard disks at the same time). then when as a last resort i tried something: i flipped both drives around, such that the PCB sides of the hard disks were now being cooled.
the internal temps of the hard disks stayed around the same however, not a single error. no problems at all.
since then ive replaced one seagate with a wd1200jb, but the other, which was giving problems, has now been running problem free for about 6 months (the drive itself is about 15 months old).
so from this i gathered that it is more important to cool the PCB side of the hard disk (which contains all the chips and controllers). if these things get hot, then your hard disks can behave unpredictably.
so as far as im concernced, heat is most definetly a factor when discussing hard disk reliability.