The fan controller senses the temperature and can vary the fan speed from off -> 1/4 -> 1/2 -> 3/4 -> full. It normally starts around 45-50 deg. C (when measured at the HDD). Typically it will only run for a few minutes in the hour.
What makes you think that the HDD has been damaged by high temperature? What are the symptoms of your HDD problems?
What component controls the fan voltage/speed please.
No idea, but I think it would be better to check the disk’s SMART data for temperature readings and disk errors before opening the box or reaching for the soldering iron.