For those participating in Apple’s public beta program or developer program, here’s a script that will make a bootable flash drive installer of Sierra for you. Of course, you’ll need to have downloaded and saved the original installer before running it on your mac for this to work.
When an installer is made available to you from Apple, the first thing to do after downloading it is to quit the installer if it auto runs. Insert your blank USB thumb drive, and make sure it’s at least 8GB (16GB recommended).
You can either run the script immediately with the installer app still in your /Applications or /Downloads folder, or you can move the installer first to your preferred location. It doesn’t make any difference to the script since it’ll ask you for the location of both the Installer and the USB drive before doing its thing. It’ll also give you an option to cancel out if you made any mistake in specifying the location or you just change your mind. The script will ask you for an administrator password as it needs elevated privileges to run the
Note the script continues to run in the background until the installer has been created. It sleeps for an interval of 10 secs between checking the job status. Since it takes around ten minutes for the createInstallMedia routine to finish its work, you could comfortably increase that sleep time 30 secs or more if you desire. The script will present you a dialog when it detects all is done:
To use the bootable installer, just pop it into a mac, reboot holding down the ‘option’ key and choose the USB drive to kick off the installation process on a partition of your choice.