If you have a Mac with a T2 Security Chip, it is easy to downgrade to the any OS Once you are booted with the installer, you can either erase your disk and perform a fresh installation of macOS High Sierra, or partition your disk and install macOS High Sierra in parallel to your macOS Catalina install. Once you have the installer ready, restart your Mac with the removable media plugged in, press and hold the Option key, and select the installer on the boot screen. How to create a bootable installer for macOS.Instructions for creating a bootable macOS installer can be found here: To downgrade your Mac by installing an older version of macOS, you need to create a bootable macOS installer on a removable media. The error message is conveying precisely the same thing to you: This copy of the "Install macOS" application is too old to be opened on this version of macOS If your Mac came pre-installed with macOS High Sierra of any earlier version, it can run macOS High Sierra.Īssuming it does, you can't run macOS High Sierra installer from within macOS Catalina (or any newer release of macOS). You need to first make sure that your model of Mac supports running macOS High Sierra.
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