I bought my new P870 with Win 8 installed in december 2012. I found USB 3.0 not working with my Agfa 64GB USB 3.0 stick.
I found warning message on Intel USB-IF xHCI USB host controller. Reinstallation of driver not possible.
After many hours searching the internet for a solution, I'm frustrated now with no success.
By the way, in USB 2.0 mode the stick is working well.
Very interesting for me: I tried to use this stick in the store where I bought my PC on other Win 8 laptop machines from other suppliers like Samsung, Asus, Acer, Lenovo ... Also no function. On Win 7 machines there is no problem and the stick is workung well under USB 3.0.
For me it seems to be a general problem from Microsoft or Intel.
What to say… in my opinion you should check if other USB 3.0 devices would work properly in connection with USB 3.0
At the other hand it could be possible there is a general compatibility problem with this USB sticks and USB 3.0 Win 9 drivers… you said that this stick did not work with any of the different notebooks preinstalled with win 8.
So it seems like this USB flash memory stick has definitely a Win 8 problem.