it doesn't need any special cooling. i have two raptors (RAID) and a WD cavier 160GB in a three drive cage with the normal fan moving air past them and they run withing spec temps.
do you have HDD SMART enabled? there is a temp sensor on all modern drives, and if you have SMART enabled, temp monitoring progs can keep track of your temps for you. so long as they are not getting up near 50C you really don't have anything to worry about. mine usually stay just a bit lower than 40C.