How to Refresh USB Ports
Devices such as external hard drives and flash drives require USB ports to transfer data with a computer. You can use the Device Manager in Windows 7 to monitor USB ports as well as USB devices attached to your computer. If your computer cannot detect a USB device, try refreshing the USB port that you attached the device to by using the Device Manager.
Click the "Start" button, type "devmgmt.msc" without quotes and press the "Enter" button.
Expand the option labeled "Universal Serial Bus Controllers" by double-clicking it.
Right-click a USB port and click the "Properties" option.
Click the tab labeled "Advanced."
Click the button labeled "Refresh" to refresh the USB port.