I have an LG External Hard drive, 1tb, it worked fine until one day. I don't know why, but it didn't show on "My Computer".(I run on Windows 8.) So I went to the "Device Manager", there it recognized it as a "USB mass storage". But in "Disk Management" it does not recognize it. But after some time in the "Device Manager" the "USB Mass storage" got an error saying
"This device cannot start. (Code 10)
The requested operation was unsuccessful."
So I plugged it on another computer that runs Windows XP, but it still did not work.
Try going to Start; right click on Computer; click on Manage; click on Disk Management under Storage on the left, it should up with a blue line. Right click on the blue line and select Change Drive Letter and Paths. This will allow you to choose a drive letter and the external HDD might show up in Windows Explorer.