There it was, Wednesday evening, and I was working hard to finish something for the day. Then I received an email from Watchman Monitoring, an essential tool used by sysadmins and consultants to keep track of Macs under their care, telling me about disk errors on my 2014 27-inch iMac’s SSD.
That was concerning, so I finished what I was doing, restarted in macOS Recovery by rebooting while holding down Command-R, launched Disk Utility, started First Aid, went to make dinner, and promptly forgot about it. The next morning, however, I checked the iMac and discovered that First Aid had failed with an error -69842. Another try (which took longer than it should have) failed with the same error. I could find no indication of what that error meant, but Apple’s support documentation was pretty clear about the next step being a reformat and restore. At some point in that process, I booted the iMac successfully again, just long enough for Watchman Monitoring to send an even more ominous warning.
Read more at TidBits.com