My WD Passport Ultra used to work with my Mac Pro as a backup drive for some weeks, but now its not recognized anymore by my Mac and I can’t see the drive in the Disk Utility on my Mac, the WD drive light is on and you can feel the drive is working but I can’t see it in my Mac, I tried to change the USB cable and run the WD on another Mac but still doesn’t work, I also downloaded WD Drive Utility App and its keep ask me to attach a supported WD drive while its connected. I have not update or upgrade the Mac OS since I started using the WD.
The most important thing is to recover the files on the WD not to replace the WD with a new one.
The most important thing is to recover the files on the WD not to replace the WD with a new one.
Have you checked if the HDD is recognized in the Disk Utility on the other Mac? I'd suggest you give it a try so that you'd, hopefully, be able to run 
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