Hi Peter,
That's quite interesting that you mention that, however i don't see how the larger 3.5" drives would get around that as they do use a 12v supply; but would still have a switching regulator to also produce 5v for the circuit board and thus creating the same problem. If anything and assuming that the 5v supply could influence the audio quality you would be better off with a 2.5" USB drive that was bus powered as in the case of the BDP which is power'd by way of a linear supply; this alleviating the issue all together. I suspect the totaldac d1-server is unable to power a USB bus powered drive due to the fact they use a low powered arm SBC. I don't wish to stir anything up, its just that it sounds like nothing more then snake oil.
Cheers,
Chris