It's an incompatibility issue. These drives are expected to be left in their enclosure, so they detect power on a pin that normally isn't used.

http://www.playtool.com/pages/psuconnectors/connectors.html

"You have to be careful about SATA power cables. Some of them are missing the 3.3 volt wire. People with older power supplies often use adapters which convert from 4 pin peripheral cables to SATA power cables. But since 4 pin peripheral connectors only supply 5 and 12 volts, the SATA connector is missing 3.3 volts (there's no orange wire). There are also a few older power supplies which inexplicably have SATA power cables which are missing the 3.3 volt wire. Currently, SATA drives rarely use 3.3 volts. That may be because there are too many people using adapters so the drive makers don't want the headaches which come with using 3.3 volts. But in the future, 3.3 volt drives may become common so you need to be careful when using SATA power cables which don't implement 3.3 volts."

Basically, the HGST drives use that 3.3 volt Pin 3 to perform the same function as unplugging and plugging back in the hard drive. So using a Molex to SATA adapter should allow for a workaround.