unhook all other usb devices.
If a slow usb1.1 device is connected to on the same internal hub as a usb 2.0 device it will cause both to be reverted to usb1.1 speeds (if you use a usb keyboard or the likes try only hooking your external drive to your front usb's or something).
Other than that you can check your bios to make sure that "hi-speed" usb compatibility is enabled. Or try a new usb cable.
If these dont work then im not sure