I am new to posting to the forums and I just happened to come across this post, so I thought I may be able to help.
I had almost the same problem. The only difference is that my (USB External) HDD would initially work, then after a couple of seconds would start making those clicking sounds and I wouldn't be able to access anything after that (I don't know if this is the case with you).
There are a couple of things that I did to "fix" this (I am not sure exactly which one of them worked, so you may want to try them all one by one):
1. Check that your AC Adapter and USB connections to the HDD are properly connected.
2. If by any chance you disconnected your USB HDD from the main power outlet, make sure that you reconnect it with the same polarity (ie, left adapter terminal to the previous plug point and the right terminal the same as well)
3. If you changed the usb port which the HDD uses to connect to your PC/laptop try putting it back into the old one (I don't know why this could make a difference but it worked for me (after doing all this)).
4. (I am assuming you have basic computer knowledge for this step) Open your USB HDD enclosure and make sure that all the connections are in place (ie, a gentle nudge in the right direction). If you want (and know enough about it) you can disconnect the HDD from the USB HDD and then reconnect it again if you want.
This seemed to work for me and I hope it works for you as well
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I know how painful it is to loose data
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