I was stuck in this very same situation for a while now and have found a way to create an install media (installing while we speak) on MacOS Catalina. A bit of Terminal magic is required, but nothing too fancy.
I will create a temporary folder on the Desktop so it is easier to inspect the structure with finder.
Steps to take to get the command running as suggested in https://support.apple.com/en-sg/HT201372
- Download and mount the image for El Capitan (e.g. double click onto the .dmg file)
- Open the Terminal app
- (optional) Change to the Desktop
cd ~/Desktop - (optional) Create a temporary folder mkdir installCapitan && cd installCapitan
- Extract the installer here
xar -xf /Volumes/Install\ OS\ X/InstallMacOSX.pkg - Change into the resulting folder, with
cd InstallMacOSX.pkg - Now you're greeted with several files, e.g.
Payload, Scripts, InstallESD.dmg - Unpack Payload with
tar xvzf Payload - Now you have the
Install OS X El Capitan.appApple talks about in the link above - One more step, move the install data to where it should be:
mv InstallESD.dmg Install\ OS\ X\ El\ Capitan.app/Contents/SharedSupport/ - Now running the
createinstallmediacommand should work when given the right paths, and this should work:
sudo Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia \
--applicationpath ./Install\ OS\ X\ El\ Capitan.app \
--volume /Volumes/USB
Change your installation volume path accordingly and be careful to pick the right one, otherwise loss of data may occur.
edit: If your installer fails with something about not being able to verify the install media, check this answer out, it helped me: https://apple.stackexchange.com/a/232016/371520
Good luck