I'm afraid that your external hard drive has most probably failed, @plz_help_lachy!
Unfortunately, there's not much troubleshooting you can do by yourself now. It would only put extra stress on the HDD and it would definitely be very dangerous for your data. Is your drive still covered by the warranty? Have you checked it?
I'd advise you again to get in touch with the HDD manufacturer's customer support for assistance. If the HDD is within the warranty period, they should be able to help you out by replacing the failed hardware, once you RMA it. They can also refer you to some data recovery companies that you can consult with, and hopefully, get your files recovered.
In the future, make sure you always backup your data somewhere else as well. Always remember that "backup" means that you have your data stored in at least two (2) locations. Moving data from your system drive to an external hard drive is not a backup, unless there is already a duplicate of the file on a different drive.
Best of luck! Keep me posted if you have more questions.
SuperSoph_WD