I'm disappointed in this "run it and forget it" thing. Others have seen this. Most probably have not. I mentioned it in Time Machine Error. Well, it continued to happen. Sometimes it would fail with a pop-up message saying, "Time Machine Error. Unable to complete backup. An error occurred while creating the backup directory." Very helpful. What am I supposed to make of that, let alone Mom and Pop or Aunt Ida? So I opened Time Machine and clicked the red "i" in a circle, assuming it meant "information." And that pop-up said... the exact same thing.
I turned off TM. I used Disk Utility to Verify and Repair. It would not verify or repair. Now, Disk Utility helpfully tells you, "Click Repair Disk. If the repair fails, back up and erase the disk."
Back up where? And why? It is bad. Why back up a bad disk. So, I have no choice but to erase it and start Time Machine captures again. Why? Why not? I am glad I routinely back up on another volume using SuperDuper! Oh, and I need to select "Change Disk..." in Time Machine and pick the same, now zero-ed out, disk. And I'm fairly smart. Again, how could Mom and Pop or Aunt Ida do this?
Help on my Mac turns up a topic entitled, "Time Machine stops backing up to external disk." Promising? No. It says to 1. Open Disk Utility and 2. Click the Partitions Tab.
Funny. No such Tab.
Apple, this rots.