The specific drive being used makes all the difference. many of the older drives generate quite a bit of heat. Even SSD. Initially when I got my 4900 I had installed and old intel 120GB SSD I had lying around, and it would get quite hot. Then I eventually replaced the drive with a Micron 240GB enterprise grade SSD and the unit now get's just barely warm to the touch. The controllers used on many of the older SSD would generate quite a bit of heat.
I'm sure many of the older mechanical SSD would get incredibly hot also. The newer drives often use better bearings with a little less friction and therefore less heat. Even on desktop drives, I had tried some regular drives in a synology NAS server and the drive temps would hover around 41C to 42C on average. When I replaced the drives (with Constellation ES.3 enterprise drives) in the same unit with the same settings, the drives would hover around 31C to 32C on average. The type of bearings used in mechanical drives is very important to heat generation.
One hint also, if I remember correctly, the 4900 used to come with a small thin rubber pad to put on the HDD to fill in the gap between the HDD and the door to avoid it from rattling/moving. Don't intall that on the whole face of the drive. Just need a small slice (about 1cm wide), and you can install it just on the opposite end of the sata connectors. That is sufficient to keep the drive from moving around, and also leaves a small gap to allow a little airflow. I know it is not much, but every little bit counts.
