Okay, I've been really scratching my head with this one.
I recently bought a new computer with the P55-USB motherboard, Intel Core i5 2.8 @ 3.8, 8GB RAM, Windows 7 64bit SP1. I also picked up a USB3 WD 3TB External HDD, but unfortunately it didn't work - the computer failed to see the drive at all when plugged into either of the USB3 ports at the rear of the machine, but worked fine in a USB2 port (albeit at USB2 speeds obviously). So I sent it back and got a replacement, but had the same problem. I now have a Seagate USB3 3TB External HDD, which will connect via the USB 3 port, but I get a message telling me that the drive will work faster if connected to a Superspeed USB 3 port, which of course it is. I've checked the transfer rate which is around 30-35MBps which suggests USB2 speeds.
I've checked the cables and the ports to make sure they have the blue tab that denotes them as being USB3 (and they do), I've checked in BIOS to make sure USB3 is enabled (it is), I've checked to make sure I have the most recent version of BIOS (I have F10g) and I've updated the USB3 drivers (ver. 2.1.16.0) and Firmware (ver. 3027).
I've also now tried a Transcend USB3 pen drive, which has the same problem as my Seagate HDD i.e. it connects but only as a USB2 device.
This really has me puzzled. Any and all help is gratefully received.