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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 "package" 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
```
Execute the following instructions to create correct disk image:
```
$ 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
```
You must then format a GUID partition scheme USB key of sufficient size (8 GB for El Capitan) named KEY in the example and 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.