If your PC is not recognizing your USB mass storage device, there are several methods you can use to find your USB key or external HDD.
What To Do When Your USB Isn't Detected
Connect your USB drive to your PC and open the Control Panel. Open the Device Manager and click on the Scan for hardware changes button. This will force your computer to search for the USB key:
If the above solution doesn't work, go back to the Device Manager and expand the USB Serial Bus controllers section. Right-click on the faulty USB device and select Uninstall to uninstall its driver:
Once done, restart your computer and let Windows reinstall the missing driver. If the problem persists, go to the Control Panel > Administrative Tools > Services. Search for the Shell Hardware Detection service and set its Startup Type to Automatic:
Repeat the operation for the Plug and Play service:
When finished, close the Services utility and restart your computer.