Once you have the .dmg file (typically named InstallMacOSX.dmg or similar):
Warning: macOS Sierra (10.12) is an older release. If your Mac supports a newer macOS, prefer the latest supported version for security and app compatibility. The steps below assume you understand the risks of running older system software.
There are two primary paths to get the DMG.
Error: "This copy of the Install macOS Sierra application is damaged" download macos sierra 1012 dmg file install
Because Sierra is old, you may see an error saying the installer is "damaged" or "expired." This is due to expired Apple security certificates. Disconnect from the internet. Open Terminal from the Utilities menu in the installer. Type: date 010101012017 Press Enter. Try the installation again.
If you can tell me what you are running, I can give you a more specific step-by-step guide .
: Before starting, it's highly recommended to back up your Mac. Use Time Machine or another backup method to ensure your data is safe in case something goes wrong during the installation process. Once you have the
and immediately hold down the Option (Alt) ⌥ key. Select the USB installer from the boot menu.
Insert your bootable macOS Sierra USB drive into a USB port.
| Issue | Solution | |-------|----------| | “This copy of the Install macOS Sierra application is damaged” | Set system date to 2017–2019 in Terminal: date 0101010117 | | DMG won’t mount | Verify checksum; re-download if corrupted | | “macOS cannot be installed on this Mac” | Your Mac is too new (Sierra lacks drivers for 2018+ models) | | USB installer not booting | Ensure USB is GUID partition map, Mac OS Extended (Journaled) | There are two primary paths to get the DMG
Clicking the link downloads a file named InstallOS.dmg . This container holds the actual macOS Sierra installer application. Step 2: Extract the Installer Application
macOS Sierra 10.12 was released in 2016 and is no longer supported by Apple. It does not receive security updates, and many modern apps (browsers, Slack, Zoom, etc.) no longer support it. Only proceed if you need it for legacy hardware or specific old software.