virtualbox.org

I'm running Windows 7 Ultimate guest and whanever I plug in a USB key it won't recognize the device and shows "unknown device" with exclamation mark under the USB controller in the device manager.
I tried to re- install the driver but it still has the same problem. Windows said the driver is correct.
can anyone help to troubleshoot that?

Has the Extension Pack been installed?

yes, and I unmount the USB key from Mac OS already.
I eventually found that I need to change the CPU count back to one (instead of 4) in order for USB devices to be recognized.
but this sound quite counter-intuitive.