WD's marketing speak states that "for each drive model, WD may use a different, invariable RPM." That means that a particular model is designed to spin at a fixed speed. AFAICT, most (all?) Intellipower drives are spinning at 5400 RPM (or 5900 RPM).
Western Digital says:
Overall, this product meets the performance requirements of most applications while delivering significantly less power consumption. If your application requires mostly sequential read/writes, then the drive will perform comparable to a 7,200 RPM drive. However, if your application performs mostly random mode operations, then the performance may drop by about 10% because of the latency time. If the Green Power product does not meet your performance needs then we have our Raid Edition line of 7,200 RPM Enterprise drives that will deliver the performance you need at the industry’s best reliability.
IF I might say please pay attention to the full detail of peoples reply when they are trying to help you he said that they vary means that they vary in speed by make model and size they won't tell you the speed but I hate to say this but intellipower is not really much different than a ordinary drive they just through a fancy name on it to make it sell better whichb is why they won't tell you anything so i know this reply is very late but my advice is don't waste your money and also if you would like it to be more energy conservant just go into your windows control panel and set your original drives setting up to make the drives disk spin stop when the drive is inactive