Some drives like WD's Green series have variable RPM to save power (and therefore heat). I don't think the OS is involved at all with controlling how fast those disks spin (so long as they are spinning at all).
At the very bottom, in small, grey text, you'll find "IntelliPower" defined as:
"A fine-tuned balance of spin speed, transfer rate and caching algorithms designed to deliver both significant power savings and solid performance. For each WD Green drive model, WD may use a different, invariable RPM."
http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-771...
At the very bottom, in small, grey text, you'll find "IntelliPower" defined as:
"A fine-tuned balance of spin speed, transfer rate and caching algorithms designed to deliver both significant power savings and solid performance. For each WD Green drive model, WD may use a different, invariable RPM."