Is there also a possibility that by inserting the HD2, I have isolated the electrical noise associated with the usb port that so many engineers talk about?
Noise is hum, hiss, or whine, in the background.
You'd know if that was what you're hearing and you would have mentioned it.
Or, USB power tends to be noisy. Digital data is immune to it but occasionally, that noise gets into the analog-side of a USB-powered DAC, or audio interface. It's usually a high-pitch whine from the switching power supply and you can get it from a battery-powered laptop because the computer has switch-mode voltage regulators and there's lots of data-switching in a computer. It's not a problem if your DAC or interface has its own power supply, and it's mostly only a problem with microphone inputs on an interface where any noise gets amplified by the mic preamps.
radix mentioned fiber - Sometimes you get ground-loop hum with an electrical connection but with an optical connection you can break the shared ground and sometimes that will fix it.
As a rule, EVERYTHING digital is OK and you rarely get subtle problems/differences. My usual (but imperfect) analogy is that one flipped bit (a 1 switched to a zero or vice-versa) in your bank account is just a likely to cause a 1-cent error or a billion dollar error. Usually when digital audio data gets corrupted the sound gets very bad, or you get no sound at all, or dropouts/gaps in the sound, or pure noise. Even resampling is normally audibly-transparent as long as you stay at "CD quality" or better.
If you can keep everything "bit perfect" you don't have to give a 2nd thought to the digital-side of things. A lot of people strive for that. But sometimes you might want digital volume control, or EQ. or maybe you need a different format, or you want to compress to MP3 to get a smaller file, etc. And to keep things in perspective, the whole process of audio production is not "bit perfect". They are mixing, adjusting the levels, applying effects, and usually rendering to multiple different formats.
I'd recommending reading the 1st post here -
Message to audiophiles... (I haven't read the whole discussion. The 1st post makes the point.)
And
Audiophoolery
And
What is a blind ABX test?