You are facing a system problem in which your computer doesn’t see external drive. And you have installed windows 7 on your system. Perform the steps given below to solve this error.
1. Plug in the external USB hard drive.
2. Right on My Computer.
3. Left click on Manage option.
4. Left click on Device Manager.
5. Expand the USB list by right clicking on it.
6. Find the USB device for your USB Hard Drive (in my case it was the one that had no description)
7. Right click on Uninstall.
8. Unplug the external USB hard drive.
9. Now Connect the external USB hard drive and let it find and install the driver
10. Now access data from it.