Passport not recognized except in Disk Utility

I have a My Passport 1TB drive (P/N WDBBEP0010BSL) that was formatted Mac OS Extended and had been in use for Time Machine backup prior to Sierra upgrade. It no longer appears in Finder or on Desktop, and cannot be accessed by Time Machine.

Drive appears in System Information and Disk Utility but volume is greyed out and shows as full (999GB) vice the roughly 500GB actually on it. Attempting to mount from Disk Utility causes the drive activity light to flash for 70 seconds before returning solid without mounting. Activity light also flashes when starting Disk Utility. First Aid in Normal and Recovery both fail. Apple had me perform SMC and PRAM resets with no success. I have tried a new cable, as well as trying to mount and force mount from Terminal. I’m not getting any unusual noises typically associated with a hard disk failure.

What else can I do to restore the drive? Is this a partition issue or a drive failure?

Hello,

Based on the information you have provided I believe this drive is defective.

I recommend you check the warranty of this drive and get it replaced.

If you need to recover the data from the drive then I recommend using a data recovery software