Before attempting a fix, verify your current file. Open Terminal (found in /Applications/Utilities/) and run this command:
shasum /path/to/your/InstallOSXMountainLion.dmg
(Replace the path with your actual file location, renaming to Lion).
The correct SHA-1 checksum for an unmodified, clean Mac OS X 10.7.2 InstallESD.dmg (directly from Apple) is: mac os x lion 1072 dmg file fixed
e3ce4016da27b96e3bd5fb2d49366295d3ff4876
If your output does not match this, your file is corrupt, incomplete, or has been tampered with. Do not attempt to install it—you’ll waste hours. Before attempting a fix, verify your current file
If Disk Utility fails, try force-extracting the payload:
cd ~/Downloads
dmg extract MacOSXLion1072.dmg -o LionPayload
(If dmg command not found, install dmg via Homebrew: brew install dmg2img) (Replace the path with your actual file location,
Then use the open-source tool dmg2img to convert the broken DMG to a raw ISO:
dmg2img -v MacOSXLion1072.dmg Fixed_Lion_1072.iso
If this completes without I/O errors, your file was mostly intact. You can now mount the .iso and extract the InstallESD.dmg manually.
For advanced users, here’s how to create your own Mac OS X Lion 10.7.2 DMG file fixed with no expiry issues.