[Atom]

Including the cumulative effect of the extra heat dissipated within the HD, which is a significant factor aiding the abrupt increase at the end. So, heat and wear escalate it's destruction near the end, and the result is often over heated transformer windings and/or resistors in the power supply, one single resistor often being the culprit for the burning smell.

I had one HD that toward it's end I could barely touch for more than 10 seconds.

A word of advice to all: today's power supplies are complicated, it's best to not even try to repair them. However, often they remained unharmed despite the "burning smell". That all depends on the design of their over voltage/over current shut down circuit. Additionally, it's also possible for the burning smell to originate within the HD alone, another reason not to count out the power supply unless the PC is dead, in which case the PS is usually at fault, aside from the usual obvious causes such as the PC being unplugged and the like.