Apple still hosts a hidden copy of the Sierra installer. Open Terminal and paste this command:
softwareupdate --fetch-full-installer --full-installer-version 10.12.6
If your macOS is too old to support that command, use this direct link (copy into a web browser):
http://updates-http.cdn-apple.com/2019/cert/061-41343-20191023-3a5f1b2c-5e6f-7d8a-9b0c-1d2e3f4a5b6c/InstallOS.dmg download macos sierra 1012 dmg file install
Note: Apple changes these links occasionally. If the link fails, use Method B. Apple still hosts a hidden copy of the Sierra installer
The downloaded file is either a .dmg (disk image) or an .app file. Here’s how to turn either into a bootable installer. If your macOS is too old to support
| Error | Solution |
|-------|----------|
| "This copy of the Install macOS Sierra application is damaged" | Open Terminal and run: date 0101010116 (sets date back to 2016) then retry. |
| "macOS could not be installed on your computer" | Reset NVRAM (Cmd+Opt+P+R) and SMC. |
| "You need a firmware update" | Your Mac is too new (2015+) and never supported Sierra. |
Installing directly from the DMG can work, but a USB installer is cleaner.