Thanks pbcopter
The reason I am asking this question is that I am having a strange behaviour with my usb3 ports (dell xps 8500 with intel drivers). If I connect my wd passport (usb3-device) to a usb2 port, I get the message that "this device could perform.....". If I connect the usb3-device to a usb3 port, the usb3-device is not recognized at all.
If I connect a usb2-device to an usb3-port, the usb2-device is immediatly recognized (no problems here). Now if I connect my usb3-device to the other (twin) usb3-port, the usb3-device is recognized (usb2-dev in one port and usb3-dev in the other). Now I may remove the usb2-device from the usb3 port and and usb3-dev that is left is still recognized. It seems that I am getting more or less the same speed (30 MB/s) regardless if I am using usb2 or usb3 port.
It seems that the usb2 devices are "activating" the usb3 ports....it looks strange!