Welcome to the community, Sumit Hotchandani!
Can you please give us more details about the external HDD? What is the capacity of the drive?
I'd advise you to try using a different USB cable while the HDD is inside its original enclosure. You can also refer to your HDD manufacturer's website and find the diagnostic tool that will help you test its health and SMART status. If the Windows version doesn't work for you, try the DOS version of the software. It will let you perform the tests without booting into any OS.
If you have access to another computer, I'd also try plugging the external drive there and see how it will get recognized.
Keep us posted with the troubleshooting. Hope this helps!
SuperSoph_WD