Kijanki,
What you say is true. However, it is my understanding the data retrieval from the HD affects the sound quality because of the serial nature of USB and the real-time nature of streaming.
Frankly, I never had a computer/HDD that allowed for this comparison to be experienced first-hand. But it's a commonly accepted "fact" by those who claim they did try it. So in the case of someone who is to buy a HDD for this application, like Zoot45, I try and steer them in that direction. In my system I do abide by this, but can't say I campared it to a USB HDD.