If all alternate attempts fail, & you've got a lot of valuable files that must be recovered, have a look @ 'SpinRite' from GRC.
Recovers data & fixes HDD errors; disk maintenance; checks cable interfaces; many deep disk checks; etc.
Self contained DOS, so functioning OS not required.
Run it once per month or so to keep the HDD at peak mechanical performance. Some success has been reported with SSD's as well, using "Level 2.
About $90 US;
30-day, money back guarantee: no questions asked.
How many software items have you seen with that?
THE industry standard.
https://www.grc.com/sr/spinrite.htm
Google for reviews.
Well worth the money.
If you decide to give this a try, read the help files on what modes do what, cause it can be a tad confusing. Whatever 'mode' you select, don't be impatient: this works slowly, but methodically, and just may pull your drive (or data) back from the brink of doom.
In your situation, I believe the external HDD must be put into a computer as the Primary drive (aka "Master") and then boot the SpinRite iSO disk. It has a self-contained DOS so it will run on it's own.
Pretty easy to use.
If it works, BACK UP YOUR DATA right away, and migrate to an CD/DVD/ext. drive. This is imperative.
If it fails to do the trick, ask for a refund: it's got a money back guarantee, no questions asked.
Downloads in about 2 minutes (written entirely in "assembly language", and fits on a CD quite easily)