time machine runs very slowly, then stops
I've just dealt with a similar issue, and hope I can help someone. I did a completely clean install of High Sierra, then Time Machine stopped working after four days.

Your error message indicates that this is a problem with the drive you're backing up, or perhaps Time Machine's index of it.

Here are the steps to take:


Check your local snapshots, and delete anything fishy - see the empty one from the day that TM stopped working:




You would do this: sudo tmutil deletelocalsnapshots 2018-09-27-003128


Try using disk utility to repair all the volumes and drives again. If it's an APFS drive you're backing up, check all of the volumes, as well as the container volume and the disk. You can try deleting all the local snapshots too, and again try repairing the volumes and disks.
If that doesn't work, I would turn disable Time Machine - go into System Preferences and uncheck automatic backups. Unmount (restart if you have to) the drive with the most recent full backup on it - we don't want to touch that. Then, remove it by clicking "Select Disk" and "Remove Disk." Try a backup then, with just the one drive. If that doesn't work, try repairing that drive again in Disk Utility.
The next thing to try would be to remove the drive that's still associated, rename it "Time Machine Backups," and then add it back. You shouldn't lose the backups that are already there.
If all else fails, I would start over - reformat this drive in Disk Utility, but not as HFS+ Journaled - that was a mistake I made numerous times. We want Time Machine to format the drive. That should at least get you going again...


My problem was that it mysteriously wouldn't copy files.

For a week, no matter what I did, the same thing would happen - Time Machine would start, go very slowly, and then stop and give me this message:



It wouldn't complete an initial backup even. I had reformatted over and over (as HFS+ Journaled), deleted local snapshots, tried repairing the backup drive and the system drive.

The infuriating fix to my problem was simply to remove my encrypted APFS volume. fsck/First Aid kept returning a problem with the container and system volumes - "mount apfs exit status 73" and "System check exit code is 78." All that was happening was that it couldn't mount the encrypted volume. After I deleted that, I was finally able to fix Time Machine.

I still had trouble though, even after zeroing out the first 300MB of the backup drive to totally nuke the partition map and EFI. Eventually I formatted it as exFAT, let Time Machine format it, which it failed to do:



And then I went back to Disk Utility and formatted it as HFS+ Journaled Case Sensitive. That's when it started working.