Welcome to the TH Community, @spunkyfugu!
Did you by any chance accidentally dropped your external HDD? Have you tried using a different USB cable as well? You should know that mechanical hard drives are pretty sensitive and can easily get damaged. It looks like you are dealing with a corrupted file system. However, whether you use the GUID partition table or the MBR one depends on the capacity of your external hard drive. Both partition tables will reformat the drive and erase the data. You can definitely try using third-party data retrieving utility but it would be at your own risk!
I'd strongly recommend you consider testing your drive using
its manufacturer's brand-specific diagnostic utility first. It will help you determine the health and SMART stats of the HDD. If there are any SMART errors present, I'd advise you to get in touch with a professional data recovery company as they are your best bet on getting any of the files back.
If you manage to get access to your files, back them up immediately!
Keep me posted with the troubleshooting!
SuperSoph_WD