Exterior hard drive not recognise by the computer. the exterior hard drive conected throug usb to

Question

the exterior hard drive conected throug usb to the computer but you cannot find it inside the computer icon and cannot be used for backing my email [e-mail] also I have another problem 4gb flash drive sombody empty it and cannot be used anymore the wollum 0 have been change please any body with a solution please help george