Take my advice if the warranty on the external drive is still in effect.
Don`t bother messing around with the external drive, simply take it back to where you bought it from if a local shop.
Or return it via the Rma procedure of the online retailer it was purchased from H_dansita.
If newly bought.
The error means that what we call the bridge board that uses a usb interface or connection to a 2.5" or 3.5" sata drive based in the external box has failed completely.
All data is lost unless you are willing to open the external drive enclosure that will void the warrant of the drive.
Where you can remove the bridge board in most cases so you can connect the drive inside the enclosure to any system that has sata ports on it and sata power connectors.
It`s a common fault and Failure on external drives made by Seagate, and WD.
Poor quality components used.
It can be even worse if the drive uses a controler board it`s self with a usb data connection or connector soldered onto the hard drives controler board also.
Make no mistake a drive that produces an I/O error message tells you the drive controler board is or may be completely non working.
If the case no data recovery by yourself will retrieve
the data lost written to the drive in question H_danslta.
As a point.
Click on the link bellow to see a rough google listing of the amount of WD external drive that have been bought and failed withing two months of use due to the exact same problem.
https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=wd%20external%20drive%20faliures%20passport%20drives