In some cases, you get the error message “Untitled is not a valid volume mount point” when you are creating a macOS Sierra bootable installer from Terminal.
This is a bug of macOS Sierra installer. The downloaded installer from Apple is version 12.6.06. All you have to do is open its Info.plist file and change the value for CFBundleShortVersionString from 12.6.06 to 12.6.03.
This can be done with a single command using plutil:
sudo plutil -replace CFBundleShortVersionString \ -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist
Once done, try to create a macOS Sierra bootable USB stick the error should be gone.
$ sudo plutil -replace CFBundleShortVersionString \ -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist Password: $ sudo ./createinstallmedia --volume /Volumes/Untitled \ --applicationpath /Applications/Install\ macOS\ Sierra.app Ready to start. To continue we need to erase the disk at /Volumes/Untitled. If you wish to continue type (Y) then press return: y Erasing Disk: 0%... 10%... 20%... 30%...100%... Copying installer files to disk... Copy complete. Making disk bootable... Copying boot files... Copy complete. Done.