Is the USB interface on your USB drive itself USB3-capable?
And are you trying to plug that into a chassis-port which is limited to USB2-capable?
If your answer is yes to both of those questions, you may wish to consider reading the article which is spotlighted on the cover of the Ubuntu Full Circle Magazine, at page 39, entitled
- USB3 HDD on USB2 Hardware
That details how I had to work thru the process to resolve the issue.