I'd strongly recommend you use the external HDD's brand-specific diagnostic tool from the manufacturer's official website and use it to check up on the health and S.M.A.R.T. data
of the drive. If you cannot find it, you can refer to some third-party testing tool suggestion from the forum
. I'd also suggest you attempt to plug the hard drive using a different USB cable and see if that will help you detect it properly.
I'm afraid that you might be dealing with a failed drive and tampering with the data using third-party retrieving tools might potentially do more harm than good.
Keep me posted with the S.M.A.R.T. results.