I am beginning to wonder if the constant fan noise of my PowerBook G4 is being caused by the harddrive. The sensor underneath my harddrive is recording temperatures at or greater than 46 C. By way of comparison, my processor is recording a temperature of 41.8 C - 42.5 C. The fans only go on when the harddrive sensor hits exactly 46 C and don't go off until the same sensor reads 44 C. The CPU and GPU temps do not change that much when the fan comes on.

I should also note that the hottest part of the computer - by far - is the left palm rest where, you guessed it, the hard drive is.

Here is my theory: Basically, I think the hard drive is a heat monster and if I replaced it with a cooler drive, I might get away with no more fan noise.

It seems to me that the only PowerBook owners that are really complaining about the fan noise are those with the newest generation of PowerBooks (Rev. D models). The earlier (Rev. A, B, and C models) all came with 4200 rpm harddrives and mine came with a 5400 rpm HD.

Since I don't care about HD performance too much, I was thinking about the possibility of downgrading to a slower HD. The biggest differences that I've found is that my HD seems to use a huge amount of wattage to read/write and when in standby.

I found a 4200 rpm Fujitsu at Newegg that uses far less wattage.

Here are the stats of the two hard drives for comparison:

Mine: http://www.hitachigst.com/hdd/support/5k80/5k80.htm

The one I want to buy: http://www.fcpa.fujitsu.com/products/hard-drives/mhv2120at/specifications.html

For each one, scroll down to where it lists the Power Consumption.

Finally my question: Will the drastic differences in wattage for the two HD's lead to a noticeable difference in heat dissipation? If I went with the slower (but larger) HD listed above would I be able to decrease the HD area by a few degrees C?

Well ... heat is pretty much governed by power dissipation, and it sounds like there's good anecdotal evidence for your theory ...

But, half a watt is, well, half a watt. It's hardly a "drastic" difference in absolute terms. Sure, the Fujitsu (in theory) consumes half the power in terms of proportional power, but half a watt just isn't that much heat.

Keep in mind that, unless you are an intensive disk user (and I don't think you are if you're considering a downgrade), it's only idle power consumption that matters a lot — your HDD will spend 95% of its time in idle mode waiting to be used, so it's idle that will govern the average power usage.

If you're willing to spend the money, it might be worth a try, but it's difficult to speculate on published specs alone...

Your point about the idle state of the drive as being predominant is a very good one. I really had not thought about that, actually. Of course, in comparing those numbers, the differences are even less. This may indicate that it is not worth it after all...

wearing latex gloves would probably make more of a difference than switching drives.

I have a 12" iBook 1.33Ghz which came with a Toshiba 4200rpm drive. I wanted better HD performance so I bought a Hitachi 7200rpm drive. Big mistake, the Hitachi 7200 is an absolute cooker.

People who say "a faster drive won't make your laptop hotter because it's only 0.5-1W more heat" are speaking without experience. From my experience, there's a massive difference in the heat between the different speed HDDs.

After 2 days of using the unnacceptably hot 7200rpm HDD (it was uncomfortable to use on my lap and it cut my battery life by about 10-15%), I bought a WD scorpio 5400 rpm to replace it with. The 5400 scorpio is much quieter, and I can actually use my laptop without burning my legs. (sometimes the fan would come on with the 7200rpm HD when it was on my desk and I wasn't even using it!)

To summarise,
4200rpm was warm
5400rpm was about 20-25% of the way to being as hot as the 7200rpm
7200rpm was too hot

There IS a difference in heat output, and I can tell you this from first hand experience. People drawing conclusions from theoretical power calculations may come to different conclusions.

To the OP, the difference between 4200rpm and 5400rpm in heat was slight, but noticeable. A good quality 4200rpm drive may be worth your money if you really need to cool down your powerbook.

Idle heat was what was the most different between the 4200rpm Toshiba and the 7200rpm Hitachi I tried. The active heat wasn't so different, probably because the Hitachi spent less time in it's higher power state due to it's higher speed.