HDD health is a great application which will monitor hard drives' temperatures.
Drive needs to have SMARTs but most modern drives have that. Load HDD Health at startup and it sits in the background. Will inform you if critical temp threshold (which you can set) is exceeded and provides a whole lot of other useful info, plus a prediction of HDD health based on its log of your drive's behavior. Freeware from PanteraSoft
I've been using it for a number of years and its a reall spiffy utility