There is resistance in the connection where the USB cable plugs into the transformer. And also where the other end of the USB cable plugs into the “Dragon Cable”. You can get better/shorter USB cables, but you will still have resistance due to the physical connectors. WD may want to consider providing a transformer with a permanently attached heavy cord, that goes straight into an integrated Dragon Cable. There is really no benefit to have a removable USB cable connecting the transformer to the Dragon Cable. Plus, it costs more to manufacture, and lowers performance. Make that cable non-removable, short, and heavier gauge wiring. The whole power supply chain - the transformer, USB cable and Dragon Cable - should be one monolithic thing, and doesn’t need to be more than a foot long. If customers need more length, they can add a standard extension cord on the 120v side of the transformer. You could even get some aesthetics person to design a nice little stack setup - the Pi case sits on top of the PiDrive case, and the transformer sits on top of both of those (color it white to match). This would probably go a long way to solving these consumer power issues.
It’s so typical that design effort is put into the complex thing, the Pi Drive itself, but it’s the simple thing, the power supply chain, that comes back to bite you in the rear. Been there, done that!