Sierra provided the first (experimental) look at APFS, a file system optimized for Flash/SSD storage with strong encryption and better file management. Optimized Storage:
Launch the native app via your Applications > Utilities folder tree. download macos 1012 sierra full
This is the most frequent error when installing older macOS versions. It is caused by an expired security certificate embedded in the installer. To fix this: Sierra provided the first (experimental) look at APFS,
Before initiating the download, ensure your Mac is prepared: ensure your Mac is prepared: