Restoring Mac mini
How to erase and restore Catalina OS
Mac mini, macOS 10.14
How to erase and restore Catalina OS
Mac mini, macOS 10.14
I, personally, used USB boot drive (I have saved 10.12 ~ 10.15 each OS individually) for this kind of process.
How to create a bootable installer for macOS
You can boot directly from USB drive (by holding "option" key) and access "Disk Utilities" to erase and re-install 10.15.
Restart with the Command and R keys held down, use the Disk Utility to erase the internal drive, and install a fresh OS.
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Restoring Mac mini