If you need a DMG file specifically to create a bootable USB (often for clean installs or use on Windows), you must usually create it from the .app installer.
This is a popular open-source tool available on GitHub that can download older macOS versions directly from Apple's servers and automatically create a bootable USB or disk image. download macos high sierra 10.13 6 dmg file
shasum -a 256 /path/to/your/Install\ macOS\ High\ Sierra.app If you need a DMG file specifically to
After downloading the DMG file, follow these steps to create a bootable installer: your current Mac is "too new")
If the official methods fail (e.g., your current Mac is "too new"), these tools fetch the installer directly from Apple's servers: