Sometimes it is necessary that you want to install an older version of macOS and therefore need a bootable USB flash drive. If you use a recent Mac with Apple Silicon CPU, this can be a bit tricky, because the disk images offered by Apple for the versions from Lion to Sierra contain an installer that you would first have to run to create a bootable USB installer with the well-known command ‘createinstallmedia‘. However, exactly this installer does not run Apple Silicon. Anyway, for versions from Lion to Sierra, there is a simple solution.

How to create a bootable USB flash drive

First, of course, you need the version of macSO you wish to install. Yo can find them here on this support page.

You also need this little shell script. The script extracts the required files from the downloaded DMG and copies them to an USB flash drive. If you are interested in the exact details on how the script works, just skip to the section ‘The script in detail’ of this article.

So, you’ve downloaded the script and put it in the same directory as the download from Apple. In the following I assume that the file is located in Downloads and the script is also called make_stick . By default, a download is automatically saved and unpacked in the Downloads folder.

Since scripts downloaded from the internet, will have set the quarantine bit and are not executable, remove this bit and ensure that the script is executable. You can achieve this in by typing:

cd ~/Downloads
xattr -c make_stick
Chmod A+X make_stick

Now launch the script with these parameters: the download from Apple, the path to your USB flash drive, usually something like /Volumes/myUSB and the name of the macOS version, e.g. "El Capitan". If these parameters contain spaces, please put them in quotation marks. Of course, you take exactly the name of your download.

./make_stick InstallOS.dmg /volumes/myUSB "El Capitan"

Please be patient, this will takes a while. There are only some minor info about the progress. After the script has finished, you can unplug the USB flash drive and use it to boot your older Mac and install macOS.

The script in detail

In the first step, the script mounts the downloaded DMG and unpacks the PKG file contained therein to get to the actual installer image called InstallESD.dmg . To avoid confusion, the mounted volume does not appear on the desktop.

Now the InstallESD.dmg is mounted, also not visible, in order to assemble the bootable USB flash drive with its contents.

For this purpose, the included Image BaseSystem.dmg is first cloned to the USB drive, since such a bootable USB drive is actually a running macOS system.

Now the entire packages that make up macOS are copied to the USB drive into the System/Installation folder.

Now follows the BaseSystem.dmg as a file, as this needed on the Mac to be installed.

Finally, the USB stick is marked as bootable.

Categories: macOS


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