[Solved] Disk Utility can't run First Aid on APFS disks (yet)

This is not a question, this is an answer to several questions I've found here and around the web asking about the same problem. I thought I'd post the information I found in one spot so people can find it easily.

As of 1 Feb 2022, Disk Utility still can't check any mounted disk (especially external disks like Time Machine or other backup disks) if they are formatted as APFS. This applies to

TLDR

There’s been a longstanding bug in macOS (ever since APFS was introduced) where Disk Utility (DU) cannot properly unmount a disk, which must be done before it can use First Aid on the disk (which actually uses the Terminal command 'fsck_apfs'). But that doesn’t mean the disk or its contents are bad, despite the error messages that DU gives you such as:

"First Aid process has failed. If possible back up the data on this volume."

"Storage system check exit code is 65."

"Storage system verify or repair failed. : (-69716)"

"Storage system verify or repair failed. : (-69845)"

"The repair cannot be performed because one or more volumes are mounted. : (-69566)"

"Unable to unmount volume for repair. : (-69673)"

Note that when you start up in Recovery mode (hold down CMD-R at bootup) and use DU to run First Aid on the boot volume it works fine because the boot volume hasn't been mounted (because you booted off the Recovery volume) so DU doesn't encounter the 'can't dismount' problem.

Since APFS was introduced in MacOS 10.13 High Sierra we have had this problem in MacOS 10.14 Mojave, MacOS 10.15 Catalina, MacOS 11 Big Sur, and MacOS 12 Monterey.

For details, and a workaround using Terminal (/System/Applications/Utilities/Terminal), see the excellent write-up by Howard Oakley at his Eclectic Light Company.

** Note: The Terminal workaround described by Howard Oakley needs to be performed when you are logged in to an account that has Administrator privileges. **