This issue is discussed in detail at :
http://en.community.dell.com/techcenter/os-applications/f/4613/t/19491324.aspx
but no dell representative is around to comment / support.
The main point of failure is:
Some devices do not work well when plugged into the left usb port of Dell XPS 13
Devices that have been tried:
USB3 WD Passport External Hard disk drive - hard drive does not even mount when connected to left USB, whereas it works perfectly on the right USB
Unifying Receiver - mouse does not work when used around 30cm away. I have tried this with 2 different unifying receiver, and the result is the same. At the right usb, the mouse works perfect
other users have also reported that their android phone does not mount on the left USB but right USB works good.
I have contacted Dell support and had even changed the daughterboard that host the left USB, but the issue remains. I hope Dell can provide a solution to this issue.