The fault may be in:
1) Computer : Check your external hard disk by using another computer. If the problem persists, it is not a fault in your computer but a fault in your hard disk.
2) Hard Disk : External hard disks are provided with some drivers when you buy them. If by any chance, you formatted the hard disk thereby leading to deletion of drivers. This is the problem I once encountered with my Seagate 1TB external hard disk.
To solve this issue:
i) Go to 'control panel' and then to 'devices and printers'. There, you can see your device also in the list but with an error symbol.
ii) Right click on your device and click 'troubleshoot'. If the drivers required are present in your computer, they will be installed and your device will be recognized. Otherwise, your computer tries to get the drivers from the internet and your device will be recognized if there is a good internet connection.
Hope this answer helps to solve your problem.