WD100EFAX has been replaced by a new part number WD101EFAX. Generally when WD changes part number, there is a change in either RPM, or Cache memory but this time its not either of this. The new drives seems identical to the old one in all specifications except one.
A weebit higher power consumption
WD101EFAX spins at the same 5400 RPM as the WD100EFAX. Despite this, the operating power consumption seems to be higher. This is evident from the following excerpt from the WD Red Specification sheet.
Our Guess : Its an Air Filled Drive
While every other manufacturer fills 10TB and above drives with Helium, WD ultrastar became the first drive to successfully make a 10TB AIR filled drive. First the WD Ultrastar 10TB drives were changed to Air filled and now it could be that Red 10TB also is Air Filled . Extra Power consumption would be due to extra drag created by Air.
Now Air filled drives are cheaper to produce ( since they don't have to be helium Sealed) and some believe them to be more reliable ( though there is no Data yet to support it), so it makes sense for WD to shift all the 10TB drives to Air when they have the technical capabilities to do so. We won't be surprised to see a WD Air filled 10TB purple also.