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Download the macOS installer in the App Store but don't install it. After downloading, the new macOS version should be located in the /Applications directory and can be used to prepare a boot medium:[1][2]

$ sudo /Applications/Install\ OS\ X\ El\ \
    --applicationpath /Applications/Install\ OS\ X\ El\ \
    --volume /Volumes/USB \

Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying boot files...
Copy complete.

On an older MacBook Pro this took 20 minutes to complete. We now have:

$ sudo file -Ls /dev/rdisk2*
/dev/rdisk2:   x86 boot sector; partition 1: ID=0xaf, active, starthead 254, startsector 2, 15794174 sectors, [...]
/dev/rdisk2s1: Macintosh HFS Extended version 4 data (mounted) last mounted by: 'HFSJ', created: [...]

Boot the machine and hold the Option key[3] and select the USB drive to boot from and the installation should begin.


We can build a boot disk with hdiutil too.

Note: the resulting disk image seems to forget to create a recovery partition[4] during the installation!

Mount the downloaded image:

hdiutil attach /Applications/Install\ OS\ X\ \
        -noverify -nobrowse -mountpoint /Volumes/install_esd

Convert the disk image to an sparse bundle:

hdiutil convert /Volumes/install_esd/BaseSystem.dmg -format UDSP -o /tmp/${VER}

Grow the sparse bundle to make room for additional packages:

hdiutil resize -size 8g /tmp/${VER}.sparseimage

Mount the resized sparse bundle:

hdiutil attach /tmp/${VER}.sparseimage -noverify -nobrowse -mountpoint /Volumes/install_build

Remove symlink and copy actual files into the sparse bundle:

rm /Volumes/install_build/System/Installation/Packages
cp -Rp /Volumes/install_esd/Packages /Volumes/install_build/System/Installation/


hdiutil detach /Volumes/install_esd
hdiutil detach /Volumes/install_build

Resize to smalles size possible:

SIZE=`hdiutil resize -limits /tmp/${VER}.sparseimage | tail -n 1 | awk '{ print $1 }'`b
hdiutil resize -size $SIZE /tmp/${VER}.sparseimage

Convert to a CD image:

hdiutil convert /tmp/${VER}.sparseimage -format UDTO -o /tmp/${VER}

Remove sparse image, rename ISO:

rm /tmp/${VER}.sparseimage
mv /tmp/${VER}.cdr ~/${VER}.iso