This happened with my previous 17" Macbook Pro. I had switched out the internal HD for a larger one, and the fan ran often. Now, I have a new 17" MB Pro and switched the 500GB that it shipped with, with a WD 1TB. The fan never, came on, except when visiting odd sites with video. I just put in a 750GB WD Scorpio 7200rpm drive this morning, and the fan is almost always on, sometimes loudly. Anyone have any ideas on why different HDs affect the fan operation and what I can do about it? Thanks for any clues...