I've had this machine for about a month and have had a USB port/driver/who knows problems since. Had this problem with both Windows 8 & 8.1. First off, all drivers are up to date as far as I can tell. There is one driver that consistently appears on the Dell's driver/download screen, Intel Lynx Point Chipset Driver. No matter how many times I download and install it, it still pops up as a recommended driver.
For the problem: All of my USB drives whether it be a thumb drive or a portable, have never had any problems when connected to my desktop. However, I consistently have them drop when connected to the XPS. The portables drop the most. This happens no matter what I try to do whether it be accessing files, listening to music, watching vids, or copying from one USB drive to another. I currently plug the drives into a USB hub as the drops happen more often when they are directly connected to either of the two USB ports. No clue why that is. Maybe it's just luck. I ran diagnostics from the Dell site without errors so no clue what the problem could be.