They are not encased in a vacume if you look at the HDD there is abreathing hole that says 'Do not cover' They have holes to allow some heat to escape.
They might only be on the older HDD's as my 13GB Quantom Fireball hase two, but I guess they are for the motors then.
Air Holes!!!! Yes maybe forthe moters but not the platters as alreadymentioned. If the slightest spec of dust got on a platter or the heads thenyou will get alot of bad sectors and could ever crash the drive. Why do you think there are very few hard disk modders. Puting in clear windows etc. Because once opened and the dust get in your doomed. And those that have modded there disk have usally only done it to old disks and if they die it doesnt matter. And those lucky enough that have a working drive after the mod may not have a working drive for a long time. The disk drives are completly sealed from the outside atmosphere. I would think larger hard disks would now come with fans attached to the outside or will start happening soon. The bigger the hotter and they cant rely on a persons case cooling system. So people dont cool. I bet we will start seeing hard disk manufactures selling the drives with fixed fans on the outside taking up 2 3 1/2" bays. my 200GB's get really hot.
yup, The motor is what makes all the heat . The thick metal casing acts as a heatsink for the motor, and heat exits the hdd mainly through the sides. Most likely we'll be seeing more and more harddrives with sides like heatsinks (not sure how to put this).. just look at the WD Raptors: http://www.westerndigital.com/en/products/Products.asp?DriveID=40
They make quite a bit of noise, but if you stick it in one of those Silent-Drive thingies it'll be about the same as a normal drive. I don't really mind how much noise mine makes, it doesn't bother me. (If you're thinking of leaving your rig on overnight, you'll probably not notice it really when you're tired enough...)
I can't hear mine at all except during a defragging and just barely then. And my rig is quiet. Although they're in the zalman holders. There's actually a superfine filter type thing so dust doesn't get in through the air hole, although I'm almost 100% sure that's solely for pressurization reasons (so if you bring your HDD in an unpressurized airplane it won't explode, like in the cargo area)