Apple has included a tool to build a bootable external installer drive with the macOS Installer application for a while now. Apple actually has documentation for this tool.
The tool is called
createinstallmedia and can be found in
/Applications/Install macOS [[High ]Sierra | Mojave].app/Contents/Resources/.
When run, the tool requires a path to an external volume or partition, which will be erased and replaced with a bootable installer volume.
Note: Secure Boot Macs with the T2 chip cannot boot from external drives in the default configuration. As of this writing this affects the iMac Pro and the 2018 MacBook Pro. But it is expected that any new Macs released from now on (as in maybe at the Apple Event tomorrow?) will also have Secure Boot.
Nevertheless, having an bootable external installer is still every useful for ‘legacy’ (i.e. non-secure boot) Macs. Also, while it not a good general configuration, it can be very useful to enable external boot on machines that you frequently re-install for testing.
While the support article covers the basics, the tool gained a new feature in Mojave which is not documented in the article.
When you run the Mojave
createinstallmedia tool without arguments you get the usage documentation:
$ /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia Usage: createinstallmedia --volume <path to volume to convert> Arguments --volume, A path to a volume that can be unmounted and erased to create the install media. --nointeraction, Erase the disk pointed to by volume without prompting for confirmation. --downloadassets, Download on-demand assets that may be required for installation. Example: createinstallmedia --volume /Volumes/Untitled This tool must be run as root.
The new argument in the Mojave is called
--downloadassets. The description is a bit sparse, but from what I gather this is download additional assets, like firmware installers and bundle them with the other installer files on the installer drive instead of downloading them on-demand during installation.
This will not remove the requirement for the Mac to be connected to the internet during the installation process but it should speed up the process quite a bit.
If you want to learn more about how to create external installers and how to use the macOS Installer app most effectively in your workflows, you can buy my book ‘macOS Installation for Apple Administrators’
Scripting OS X