(a revelation since I previously reported Time Machine wasn't backing up every hour)
It was after 10 last night when I noticed that the most recent Time Machine backup was shortly after 2pm. I then noticed why that might have been the case: the icon for the Time Machine volume had disappeared from the Finder window sidebar. But the other volume on that drive was fine.
Disk Utility saw the TM volume, but it was nameless. It reported a problem but could not fix it. This, I thought, was the reason I reported here, earlier, that TM hadn't been backing up every hour.
Ultimately, I remembered having read somewhere that Disk Warrior has been successful in repairing volumes that Disk Utility could not. As there are relatively few TM backups since recently wiping my internal drive and updating it to Sierra (earlier backups are of the environment under Mountain Lion), and I had no need to restore anything from either environment, I figured I'd give Disk Warrior a shot.
You might recall that I had more recently used iPartition to delete an old and unused partition on the TM drive and added that space to the TM partition. That operation went without a hitch. So I can't say whether that had anything to do with the error that Disk Utility reported but couldn't fix.
Disk Warrior did its thing. It found a number of issues and fixed them all, and I was thrilled when the TM volume icon appeared once again in the Finder window sidebar.
Interestingly, though, when I clicked on the icon, the Finder window displayed no contents. So I launched Disk Utility, which now sees the volume, and it surprisingly found several errors and fixed them. Obviously, there was a discrepancy between how Disk Warrior and Disk Utility saw things.
So now, Time Machine has been busy for the past 70 minutes, apparently doing a complete backup (backups done prior to this issue took only several minutes). When it's done, I'll check whether it recognizes the previous backups.
I'm glad I had Disk Warrior and that it was able to resurrect this volume.
Oh, and a final note to users of ClamXav: I can't say for certain, but it appears that the ClamXav Sentry slows down the backup process and even when manually copying data between volumes. Things seemed to pick up when I quit ClamXav Sentry.
I am a proud Aspie. I wonder about and see things in a very literal, logical and pragmatic manner. Thus:
"The only spin that is of any real value is that which follows the rinse cycle."
MacMini 2.5 GHz Intel Core i5, 16 GB RAM, OS 10.12.4