>>4 is right.
PROTIP: things get bigger when you apply heat. this applies moreso to metals, which are highly sensitive to changes in heat. this heat also burns off the lubrication that spins the drive, and since the platters are expanding, making the gap between head and platter that much thinner.
good news: hdds are almost entirely made of metal, making cooling easier. as long as your drives have some breathing space, and you dont but them say, directly over the video card, and avoid cramming everything into a tiny, poorly ventilated case, your HDD should work for well over 5 years.
the most likely cause, second to heat, is when the drives park and spin up, because thats when the head actually leaves the platter to go park (that way you dont kill a drive if you move it with the power off). this, combined with a warm or even hot drive, can cause a head crash, which is the nastiest of all the hdd crashes, because it almost garuntees some level of data loss.
mostly, there are three types of crashes:
platter seizeing(when the axel the platters are on wont let the platters spin) is also nasty, but that can ressurect it long enough to get your data off the drive by freezing it.
common cause: old age. lubrication wears out. heat warps the drive. anything that can move can corrode and wear out.
electric shorts(when the green PCB on the bottom of the drive dies) on the board are the least data-damaging, since those boards are, in an emergency, replaceable.
common cause: power surge/brownout/user error. this ones self explanitory.
head crash(the head on the tip of the actuator actually collides with the magnetic platter that holds your data at scale speeds of up to 300 miles per hour. not pretty.)
Common causes: abusive handling, drop while power is still on, old age (platters can loosen up over time, causing the opposite problem as platter seizing)