Try this 1
Follow the below instructions step by step and check if your computer is able to
detect the external drive.
1) There are some external drives that wont detect at first instance, all you have
to do is try different USB ports. Try connecting the external drive to the USB ports
that are available at the back of your CPU. If the problem still exists then follow
the next step
2) Connect your external drive --> Click on Start --> Run --> Type devmgmt.msc
and press Enter. "Device Manager" will be opened. If you notice any yellow mark
then right-click on it --> Select the option "Update" and follow the instructions.
3) If there is no yellow mark in the device manager then click on the white area in
that device manager window. Now click on "Action"--> Scan for hardware
4) If the problem still exists then click on the below link and follow the instructions
5) If the problem still exists then try to update the USB drivers.
6) Check the USB data cable also.