APFS is almost 4 years old, and set out to solve the problem of accumulation of minor errors which has plagued HFS+. Has it?
fsck
Why you should always protect backup storage with a UPS. Should you also rebuild its directories every month? And when to archive rather than back up.
Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Disk integrity checks appear to have been dropped from Safe Boot. OpenDirectory databases are rebuilt. With a full listing of all blocked extensions.
Start up in Safe Mode: should it start by checking and repairing your boot disk, or should you do that separately in Recovery Mode?
Third party tools for checking and repairing APFS are still very limited. Is it better to use fsck to check and repair APFS?
Single-user mode to run fsck to check and repair your boot disk became tricky with Fusion Drives. With a T2 chip, it’s not practical in its traditional form.
Contains binary data on the last integrity check made using fsck on a disk image. Can be deleted to enable some images to be mounted.
You have double-clicked on a disk image, and macOS refuses. Here is what you can do to get that image to mount, even when it’s damaged.
Your disk’s journal aims to prevent disk errors which can occur after a crash. Here’s how to use it to protect your Mac’s data.
The Eclectic Light Company 