I have a Dell XPS 12 9Q33 (i5-4500U,8G,256G SSD). Aside from the image retention issue which is confirmed by lots of XPS 12 users, mine has one malfunctioning USB 3.0 port.
You know that XPS 12 has two USB 3.0 ports. One of them is next to the mini display port, and this does not support PowerShare function. The other one does support PowerShare, and this is the one with problems.
When I plug my WD Elements 1TB USB 3.0 external hard drive to this port, it randomly disconnects during the process of large file transfer, either reading from or writing to the external hard drive. (I believe reading from it causes more disconnection)
I did a lot of things to narrow down this issue. First I checked if my WD external hard drive is defective. I ran a thorough diagnostic, wrote zeros, and used it to do the same job on several other systems, including this XPS 12 with the USB port that does not support PowerShare. Under all these circumstances no random disconnection was observed. Afterwards, I plugged different external hard drives to the problematic USB port of XPS 12, and they also showed random disconnection (including USB 2.0 and USB 3.0 external hard drives).
To rule out the possibility of power settings, I toggled off "USB selective suspension" in the power settings of control panel; not effective. Actually, the fact that one of the ports is working flawlessly while the other is not is enough to draw the conclusion that the port with PowerShare is not performing well.
I talked to Dell Support about this and my motherboard got replaced, but the issue is still there. I am asking if the two ports has some differences in the fundamental design, such as power voltage or something, that caused this? Any other potential reason/advice/suggestion on this topic is welcomed.