@classithongs: I bought a 1TB Western Digital Green for my Mac Pro and it's been running about 5?C cooler than the 250GB Seagate that came with the Mac. Whether that means that it's using less enegry or not, I can't say, but at least it's not being turned into heat... :up to the 1TB WD Green.
Regarding the Seagate drive spinning/being on all the time: I've had my 500GB Free Agent hooked up to my 622 for the last 7 months with no hiccups at all. I ran the software patch to prevent the drive from shutting off when idle (per the recommendations on this board) and it's been a solid performer. I should have shut off the LED light bar, though, since it's pretty obnoxious in a Home Theater environment. Maybe some black electricians tape...