The Mac-maintenance task that has taken care of itself for most of the last four years brought itself to my attention Wednesday, and I wish it had not. Two days of troubleshooting later, I think I once again have a working backup routine–but I still don’t know what went wrong here.

My first hint that Apple’s Time Machine backup system had shifted out of its usual orbit was an error message Wednesday night reporting that my backup volume had become read-only, making further backup cycles impossible.

The drive in question, a 2-terabyte Seagate portable drive that I’d bought in 2018, seemed too young to be suffering from disk corruption. Especially since other partitions on this hard drive remained readable and writeable.

So I opened Apple’s Disk Utility, selected the Time Machine backup partition, and clicked “First Aid.” Several minutes later, this app returned an inscrutable, no-can-do result:

The volume Time Machine backups could not be repaired. 

File system check exit code is 8.

Well, then.

Disk Utility’s help was of no help, reporting “No Results Found” when I searched for that error message and shorter versions of it. Googling for “check exit code is 8” yielded nothing at Apple’s support site (a fruitless result confirmed by Apple’s own search) but did surface a data-recovery firm’s explainer that this was “one of the most frustrating file system errors to encounter, and it is difficult to know if you are experiencing a logical or physical fault on the hard drive.”

Trying to repair the volume a few more times with Disk Utility–a suggestion in a Stack Exchange thread that seemed worth testing–didn’t yield a better outcome. An attempt to copy the entire Time Machine volume to the partition that I’d created on this Seagate drive last year to usher my data from my old iMac to my current Mac mini stopped early; Shirt Pocket’s SuperDuper app was less informative than usual, saying it “Failed to copy files.”

Then I realized that I was looking right at a short-term answer: wiping that no-longer-needed iMac disk-image partition, then making it my new Time Machine backup volume while leaving the old Time Machine partition alone. After a timeout to unplug the drive and then plug it back in, without which Disk Utility would not reformat the partition, this fix seems to be working. But just in case, I’ve also plugged a 1-terabyte SSD into my Mac mini as a backup to my backup.

It would be great if Apple would provide clearer explanations and more usable fixes to disk errors like this. But considering that Time Machine’s starfield file-restore interface hasn’t changed since it debuted in 2007, I will not stay up late waiting for those updates.