First Aid to APFS USB Drive Workaround

I was confused; two issues here. First, on the iMac Pro the Command-S did result in a different video display during login where an extensive log of startup showed; the result, as stated, really still was in "normal" mode. My mistake and confusion -- sorry!

The actual way to get the FirstAid to work was discovered a bit by accident, but it seems to work reliably. This is for the disc itself; haven't experimented with the container.

1). The external USB (MyBook1 in this case) drive was connected to power and received line power when the rest of the iMac was booted.

2) Brought up the Disc Utility application and had it showing all external discs.

3) Brought up the terminal app and prepared it for repairing the disc by typing

hurtw@Music-iMacPro ~ % diskutil repairVolume MyBook1

but not yet entering "return" to execute the command.

4) Connected the USB drive to a USB port (in my case a switchable USB hub)

5) Waited for the Disc Utility to show the presence of MyBook1 in greyed out form -- not yet mounted.

6) Immediately either requested Disc Utility to do first aid on MyBook1; since it was not yet mounted the first aid worked as the previous screen shot showed.

I tried the same from the terminal -- see attached screen shot. The timing on all this seems a bit flakey, but in general it seems to work.

It appears that the macOS unmount command will not fully unmount a drive so first aid fails, but if you catch it BEFORE the system mounts the drive then the first aid or repair will work. Conclusion: Fix the macOS unmount command so it leaves the device in the same state as when initially connected to a USB port

Attempted to paste the same thing from the terminal using the diskutil.

Screen shot of first aid on Western Digital external APFS USB drive; seems to work immediately after powering on the drive and with it unmounted; system in single user mode; perhaps Apple should examine why it works in single user mode and not normally: