External disk does not show in "diskutil list" after premature eject, what can I do?
Reading the man page of diskutil, I found a command that I thought might provide at least some diagnostics on what was wrong:

diskutil activity


When you run this command, it will start monitoring all disk activity and report back to you until you stop it again.

I ran this command and switched off the bricked external drive. Nothing happened.

I switched the external drive back on while the command was still running. Now, I saw the following sequence of events repeat several times (leaving out the information that came with the events):

***DiskPeek
***DiskAppeared
***DiskMountApproval


and finally a few times

***DiskDescriptionChanged


followed by

***DAIdle


which was also the last event before I switched the drive back on.

This seemed promising, so I took a peek in the Finder and lo and behold, both partitions of the drive were there!

This was my first attempt to mount the disk while running diskutil activity and the third attempt in total since the problem started. I don't know whether diskutil activity actually fixed the problem or that this was just my "lucky try", but if you run into the same problem as I did, at least this is something you can try!