Certainly, if you're booting from a Safety Clone, you can't unmount the original volume -- after all, your Home folder is hosted there, since the user files are shared.
But -- if you're booting from a "regular" copy (as I thought you were -- that is, "Backup - all files"), there's no reason for the main volume to be busy as long as the backup is named the same...
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--Dave Nanian
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