External backup drive won't mount

I am using a MacBook Air, OS 15.6. I have been using a WD Elements 5TB external HD for years as my backup disk. Suddenly it will not mount. The Elements is plugged into a LENTION USB hub which plugs into one of the macbook's Thunderbolt ports.

When I check with apple System Report, the Elements shows up when it is plugged in and does not show up if I unplug it from the hub — so the computer is correctly detecting it. (see attached screen shot)

When I open Disk Utility, however, if the Elements is plugged in nothing loads into Disk Utility at all. If I unplug the Elements and restart Disk Utility then the MacBook disk immediately shows up.

Why would an external HD show up in System report but not in Disk Utility and — more importantly — how can I get this Elements to mount again?

I also ran (diskutil list) on Terminal — see attached.