Drives in large part sink their heat off through direct metal-to-metal contact. I've seen cheaply designed drive mounts that provide very
little surface area contact for the HD. Such cases by the time I see them are sometimes well on their way to killing a second HD. I've also noticed many poorly (cheaply) designed "anti-vibration" mounts that basically put rubber or some other soft material up against the drive's sidewalls. The drive with these devices has no metal-to-metal contact at all. Today's drives do run much cooler and so metal-to-metal is not as critical for longevity as it used to be. But if your goal is to simply have the drive run cooler, you'll want to maximize the surface area for metal-to-metal contact, in addition to mounting it lower.
In my case, drives are mounted to the bottom and another has a fan on it. Both solutions have always worked well.http://www.sudhian.com/index.php?/forums/viewpost/840901/