Create an El Capitan rescue USB using a Modern M1 Mac I am trying to rescue my dad's 2008 iMac. It is running Snow Leopard but the most recent OS it supports is El Capitan. It won't boot past a spinning progress bar on a grey screen, and will not boot into Recovery Mode or Safe Mode.
It seems to me next step is to try a bootable USB stick. I am running into walls trying to make the stick from my own computer, which is an M1 MacBook Air running Big Sur.
An El Capitan .DMG is downloadable directly from the Apple Support page, which provides instructions on how to make a bootable volume, with a major caveat On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg
I used a utility to extract the Install OS X El Capitan.app file from the .dmg and then ran the terminal command from that support page:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyUSBVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app 
But this yields /Applications/Install OS X El Capitan.app does not appear to be a valid OS installer application.
When I google this error some results indicate it is due to an expired file, but changing my system date to 2016 does not fix it. I assume the real problem is a system incompatibility.
Surely there is just an ISO I can get and write without all these hoops?s
Start by downloading the El Capitan dmg from this link : https://support.apple.com/en-us/HT211683
Then open the dmg and copy InstallMacOSX.pkg on the Desktop.
Then, from the terminal (Applications/Utilities) un-compact the InstallMacOSX.pkg file in a directory (Installer for example) which will be created by the following pkgutil command:
pkgutil --expand ~/Desktop/InstallMacOSX.pkg ~/Desktop/Installer

Then position yourself in the InstallMacOSX.pkg &quot;package&quot; created by the pkgutil command
cd ~/Desktop/Installer/InstallMacOSX.pkg

Then un-compact the structure using the tar command:
tar -xvf Payload

Finally move the InstallESD.dmg file created by the tar command above to the Desktop
mv InstallESD.dmg ~/Desktop

You must then format a GUID partition scheme USB key of sufficient size (8 GB for El Capitan) named KEY in the example and execute the following instructions :
hdiutil attach ~/Desktop/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_app
hdiutil convert /Volumes/install_app/BaseSystem.dmg -format UDSP -o /tmp/Installer
hdiutil resize -size 8g /tmp/Installer.sparseimage
hdiutil attach /tmp/Installer.sparseimage -noverify -nobrowse -mountpoint /Volumes/install_build
rm -r /Volumes/install_build/System/Installation/Packages
cp -av /Volumes/install_app/Packages /Volumes/install_build/System/Installation/
cp -av /Volumes/install_app/BaseSystem.chunklist /Volumes/install_build
cp -av /Volumes/install_app/BaseSystem.dmg /Volumes/install_build
hdiutil detach /Volumes/install_app
hdiutil detach /Volumes/install_build
hdiutil resize -size `hdiutil resize -limits /tmp/Installer.sparseimage | tail -n 1 | awk '{print $ 1}' `b /tmp/Installer.sparseimage
hdiutil convert /tmp/Installer.sparseimage -format UDZO -o /tmp/Installer
mv /tmp/Installer.dmg ~/Desktop

Here you have to plug the USB key named KEY, then :
sudo asr restore --source ~/Desktop/Installer.dmg --target /Volumes/KEY --noprompt --noverify --erase

Test the key and if ok, delete the working directories and files of this operation from the Desktop.