With the recent release of a new Mac operating system, macOS Mojave, it’s worth digging out and rehashing the instructions to create a bootable installer.

I carry around an external drive with me with multiple installer partitions on it. I bought myself a LaCie Rugged Mini 1TB hard drive and carved out a number of 10 GB partitions to fill up with installers. I initially created 8 installers in total, 7x 10 GB for installers and the rest for Data.

I’ve got installers going back as far as Mac OS X Lion. I’m going to be wiping the Lion installer and I can’t remember the last time I used it, so as things usually go, I’ll urgently need a Lion installer at some stage in the next two weeks.

Anyway, to cut to the chase – after you’ve downloaded the macOS installer from Apple, you can delve into the App bundle and Apple have helpfully put together a script that will create the install media for you, using the downloaded installer.

With the Mojave installer downloaded, drop into Terminal and run the createinstallmedia command like so (assuming you’ve got a partition on a disk called Untitled that you don’t mind erasing)

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled

You can also pass it the –nointeraction flag on the command line if you’re sure you want to go ahead and don’t want to be prompted about anything else.

For macOS Mojave, there’s a new flag, –downloadassets which, according to the installer, “Downloads on-demand assets that may be required for installation”. I’m not sure what this does yet, I’ll post more after I’ve had a chance to test it. Presumably it caches optional or machine-specific assets to disk, leading to a faster installation. In my initial testing, this appears to download an extra ~250MB for the Apple update 041-08711 which contains BridgeOS installer files – BridgeOSBrain.pkg and BridgeOSUpdateCustomer.pkg. BridgeOS is the cut-down operating system that runs on the T1 and T2 processors in the Touch Bar in new MacBook Pros, as well as the new iMac Pro.

Since macOS Sierra, you no longer have to give the –applicationpath argument on the command line, pointing to where you have downloaded the installer app, it can now work this out for itself.

(Those flags in the text above have two dashes at the start, not an em dash. WordPress is helpfully correcting two hyphens and showing it as an em dash so copying and pasting will only work from the code block where they’re shown correctly)

Source: How to create a bootable installer for macOS – Apple Support