For the benefit for anyone who is still having this trouble since 2013 (seriously?), below is my personal update. (For previous discussions, please search "U2713H REV4 USB 3.0 not working.")
The USB 3.0 ports and card reader of my 5th replacement U2713H (a refurbished A03) received Aug 2013 failed again in about 4 weeks time. Figuring that Dell is probably just switching dead units with other not-yet-dead units, I decided to do the wise thing - bought a separate USB hub and card reader and ditched the built-in ones. Not a single problem since then.
If the problem still bother you I suggest you do the same - save you tons of time.
That being said, I just got the unit replaced again, to see if the issue is finally fixed by Dell. I now get a U2713Hb Rev A07. So far it has been running fine for 2 weeks.
I will update again if anyone is interested...