Operating system
If I go back to my original operating system will I still have all my files
iMac 21.5″, 12.2
Question marked as
If they're on the volume you're downgrading, only if you've previously backed them up.
If they're on another one, or you install an older OS to a blank one instead of downgrading the main one, yes.
In any case, you should have a backup.
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