well, new harddrives dont care how you mount them....
if its a new drive, i think you can pretty much mount it how you please. Thats the case with maxtor drives anyway, and ibm.
 
//Silent

I wouldn't have thought that there was a topside and a bottom side as far as hard drives were concerned - I thought that each disk platter had a head for each side?
 
Of course, I may be wrong, and just talking rubbish (wouldn't be the first time!)...
 
AndyF

older harddisks, especially seagate, had the middle not fixed, but only laid upon the center. So it was spinning on the housing when mounting upside down. They couldn't even be mounted on side.
But they were more silent when mounting upside down. But still too loud.
Today, it shouldn't matter for this point. They're fixed tightly. But the head is very nearby the platter's surface - i guess it could get very close to the one side when mounting the wrong way. And thus increase the possibility of a headcrash.
As the other side is farer away, access times could be slower due to more retries. Just benchmark it normal and upside down for to know if this is still a matter or not.

Just out of curiosity, why are you thinking of mounting them upside down?
 
I know heat rises, but if it's for cooling, you're probably better off getting proper drive cooling bays or a couple of fans blowing air in the general direction of the drives.

Mostly it does not matter how you orientate modern drives. However it may have an effect upon shock absobing. I know this applies to the Quantum Atlas 10K II.
 
If you really want to know for a particular drive visit the drive manufacturers site. They have information as to how the drive should be mounted and used.

The reason I'm doing it is because of the way the connector on the drive is positioned. It's an IBM 4.5gig U2W drive. Trying to mount it normally requires me to twist the cable around into a big mess to connect with my cdrom drive. Mounting it upside down removes this problem. I've had it like this for a few days, and haven't noticed any performance problems.

Hot air rises.
 
One side of you hard drive has a protective cover, yes?
 
Well....

From what I've found new drives can be mounted in almost any way. It is however recommended you keep them mounted horiztonally/vertically and not angled.
 
Most drives have an exposed PCB on the bottom side of the drive, if you mount it upside down you'll more than likely get dust build up on the PCB. It's fine for other PCB's in the system (i.e mainboard, vid cards etc), so I don't see how it would be different on the HDD.