I started paying attention when one summer, long ago, I most likely fried a 120 GB Maxtor in my extremely insulated Sonata 1. Those 100-200 GB drives would be close to the 35 °C mark, as I recall, though much in my setups and monitoring behaviour has changed since then.
This Green easily goes up to 45, even with active cooling, if placed in a restricted, sideways-airflow cage like the Define Mini's. And I don't even have a graphics card in here! Either the drive is hot (judging by wattage it's not), or the cooling is just not that effective. In fact I suspect, at the risk of sounding paranoid, that the space requirements of the bigger, multiple platters and their mechanics, combined with weight management and cost savings, may have eaten away at the capacity that modern drives have for cooling.
Since this is all so case-dependent, pardon the pun, and due to my personal history, I feel it's better to be safe than sorry with the active cooling. People do need educating on picking the proper fans, though - no harm done if they were to run a 650 or 800 RPM fan at 12 V or 100%.
PS. The Scythe Quiet Drive looks like it would be efficient at conducting the relatively small amounts of heat that HDDs output, at least moreso than air, so I'm not surprised if the Samsung stays low in there. It is enclosed, yes, but enclosed in an efficient conducting material with a large surface area - exactly what you need for passive cooling.
PPS. I should point out that the workspace has a high ambient of 24 to 26 °C (with a +/- 2 °C error margin, according to the $5 thermometer's specs), but that is to be expected in a small-ish space with powerful electronics.