The flash drive will be completely formatted. Back up any existing data on it. Step 1: Format the USB Drive Insert the USB flash drive into your Mac.
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 prefer not to use the command line, third-party tools like iBoysoft DiskGeeker or EaseUS Partition Master simplify the process.
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: Double-click the .dmg file to open it. You should see a .pkg file or an installer window.
is downloaded, you cannot simply copy it to a USB. You must extract the application: Double-click the downloaded file to mount it. Open the mounted disk image and double-click the file inside (e.g., InstallMacOSX.pkg Follow the prompts. This will install the OS; instead, it installs an app called "Install macOS Sierra" Applications 3. Create a Bootable USB Drive
The file will download as "Install macOS Sierra.app" or a DMG file. B. Third-Party Official Repositories The flash drive will be completely formatted
OS X Lion 10.7.5 or later is required to upgrade directly Part 2: How to Download macOS Sierra 10.12 DMG File
Follow the prompts. This will install the OS yet; instead, it extracts the "Install macOS Sierra" application into your Applications folder. Step 4: Install macOS Sierra
A bootable USB allows you to wipe your hard drive and install a fresh copy of macOS Sierra. This is useful for troubleshooting or speeding up an old Mac. Because Sierra is old, you may see an
Troubleshooting
However, it is crucial to note that Apple ended security updates for macOS Sierra in 2019. While it runs on older Macs, using an unsupported OS connected to the internet carries security risks.
This happens due to a corrupted download or an incorrect system time.
Apple officially released macOS Sierra 10.12 in September 2016. It introduced Siri to the Mac, brought Universal Clipboard, and offered optimized storage. If you need to revive an older Mac, set up a virtual machine, or test legacy software, you still need a clean macOS Sierra installation file.