"The cause is a new SATA specification which includes the ability to disable power to the hard disk. When you look at the SATA power connection on the back of your hard drive, there are 15 pins that make contact with your power supply. It's the third pin that delivers a 3.3V signal that disables the drive. What we need to do is prevent that third pin from making contact with the power cable."
Some hotswap harddrive bays use this feature, definitely more common in enterprise scenarios or in USB HDD enclosures.