We cannot simply drag the DMG to the USB. We must use Disk Utility’s feature or Terminal.
You can download the installer directly from the Apple Support Website .
openssl sha1 /Applications/Install\ OS\ X\ Mountain\ Lion.app/Contents/SharedSupport/InstallESD.dmg download os x mountain lion bootable usb
sudo asr restore --source /Applications/Install\ OS\ X\ Mountain\ Lion.app/Contents/SharedSupport/InstallESD.dmg --target /Volumes/USB --erase --noverify
On older Macs already running Snow Leopard (10.6.8) or Lion (10.7), you can still find it under the "Purchased" tab of the Mac App Store . We cannot simply drag the DMG to the USB
: If you previously purchased it, you can redownload it from the "Purchases" tab in the Mac App Store. Extraction : Once downloaded, if you have an installer, right-click it, select Show Package Contents , navigate to Contents > SharedSupport , and locate the InstallESD.dmg 2. Prepare Your USB Drive You need a USB flash drive with at least of capacity.
Apple includes a built-in tool within the installer package to create bootable media. We will use the Terminal application to execute this tool. openssl sha1 /Applications/Install\ OS\ X\ Mountain\ Lion
| Issue | Solution | |--------|----------| | "This copy of the Install OS X Mountain Lion application can't be verified" | Set system date back to 2012–2013 via Terminal ( date 0101010113 ) before running installer. | | USB drive not bootable after restore | Ensure the USB partition scheme is , not Master Boot Record. | | Missing InstallESD.dmg | The .app may be incomplete. Redownload from Apple ID purchases. | | Restore option grayed out in Disk Utility | Select the drive (top-level external entry), not the volume name below it. |
This comprehensive guide will walk you through everything you need to know to files, create the installer drive, and get your Mac up and running.
You now have a bootable USB installer for OS X Mountain Lion. This tool is essential for performing clean installations on legacy Macs without relying on internet recovery (which may fail due to outdated security certificates). Always back up data before proceeding with installation.