Update: nothing happened for almost half an hour but I left the drive connected and terminal.app running. This message then appeared on the job stream: Last login: Sun Mar 6 18:44:23 on ttys000
Johns-New-iMac:~ johnlawn$ sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Password:
Ready to start.
To continue we need to erase the volume at /Volumes/MyVolume.
If you wish to continue type (Y) then press return: y
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
The copy of the installer app failed.
.Johns-New-iMac:~ johnlawn$
Johns-New-iMac:~ johnlawn$
Johns-New-iMac:~ johnlawn$ The copy of the installer app failed.
-bash: The: command not found
Johns-New-iMac:~ johnlawn$ .Johns-New-iMac:~ johnlawn$
-bash: .Johns-New-iMac:~: command not found
Johns-New-iMac:~ johnlawn$ Johns-New-iMac:~ johnlawn$
-bash: Johns-New-iMac:~: command not found
Johns-New-iMac:~ johnlawn$
Johns-New-iMac:~ johnlawn$