Well at I own 3 usb hard drives
I did not say error free I said error tolerant. USB does have some issues since it's asynchronous and this can certainly cause problems in that the music drops out. That said if they digital signal gets through the only thing I can see being an issue is the jitter in the digital signal.
Jitter could be the cause but I'm not so convinced because some "amazing" transports have jitter problems and other so so transports have great jitter. Also tests on the audibility of jitter are very suspect. Another possible source is some kind of DSP going on in the transport which of course can make a difference.
I'm not saying that they don't sound different but that I don't see why they should sound different. They are taking digital data and delivering it, there are lots of devices that do this very well ranging from digital cable to cell phones and I don't see why it will take 2 grand to do this with CDs.