Exactly my question. The WD HD uses is a USB 3.0 so the hd end is different from the usual USB cable that is used from player to DAC or Computer to DAC.
My understanding is that computer USB ports produce a lot of noise and so the theory is that a better designed usb cable will help eliminate or reduce the noise.
I don't know if the same applies when just using an external hd whether Solid State or otherwise. I'm sure that probably makes a difference as well.