P8Z77-V LK : 2 of 4 USB 3.0 ports not working

Hi all,

I just built a computer with the P8Z77-V LK, and now i see that 2 of the 4 USB 3.0 ports don't work when I'm in Windows 7 (64-bit). When I plug my external HDD into either of the 2 slots, it lights up, but I can't access it. The mouse and keyboard work in these same slots in UEFI, but not in Windows. I've installed, uninstalled, and reinstalled the USB 3.0 drivers using the DVD, but it's still not working properly.

Does anyone know how I can fix this?

Thank you.