Conflicting Disc Utility Details - Failing Drive
Disc Utility isn't always the best tool for something like this.
I would probably throw something like this at TechTool Pro, Paragon Hard Disk Manager or DiskWarrior* for a good 'second opinion'. 

I would also be careful &amp; make sure I had a good clone of it first - Carbon Copy Cloner

*DiskWarrior is extremely good with Time Machine drives, as they're always HFS+, but can't handle APFS &amp; doesn't deal with the entire drive like the others, only partitions.
I've had TTP fix things DW couldn't &amp; HDM fix drives containing Windows partitions that the others couldn't - so there is no clear 'winner' between these three.
TTP contains SMART monitoring software too.

If I could only have 2 of the 3, I'd go TTP &amp; DW, probably in that running order.