A few points from my pc days and some from my audio days reworded:
1. External hard drives not in the full size 3.5inch form factor with external power will require power from your computer, power drain + spinning motor = lovely oscillating power requirements. The cpu and ram can do this all day because they have much more advanced voltage controllers.
2. Poor power supplies in computers can cause "noise" through the motherboard just by moving the mouse and having the data travelling parallel to the usb ports data lanes. We have all experienced this before with older computers i am sure. All of this data goes through PCIE lanes these days which offer MASSIVE performance but are used to having bits double checked at the same massive speeds, something most usb controllers couldn't do if they wanted to.
3. Windows was never meant as a listening station, even wasapi has its flaws as we all know.
4. Using a Linux music server can negate any negative io/noise/ kernel issues (alsa + kernel + player). All distributions offer real time kernels, and with a few tweaks you can set selective real time priority for alsa only (the audio driver/layer).
5. Never expect a laptop to provide clean power to a dac regardless of other devices attached. It's one of the bad things that hasn't changed (yet).
6. PC's are only JUST getting specialised USB ports for DAC's. Call it what you will, but testing has shown they are cleaner regardless of load and even offer higher output in general in case your DAC is a little greedy.
In the end though, technology has attacked all of these to the point that with a little research you can avoid most if not all of them. That of course requires money though :(