"Hot" may be right...
I had two 360gb drives stacked on my new dual core Gateway and the heat fried one of the drives in only 6 months causing me no small headache in data recovery. If they are stacked with less than a 1/2" space between them, consider refabricating the hard drive holder to keep them further apart.
I learned the hard way...big HDs were not meant to operate too close to other big HDs.
;-)