You can exclude all system files from Time Machine's backups, but it's not exactly obvious how to do so. Open Time Machine's System Preferences panel, click on Options, click on the plus sign, select the /System folder in the dialog box, then click Exclude.When you do so, an additional dialog will appear, asking if you wish to exclude all other system files and applications, or just the chosen folder. Click Exclude All System Files, and a 'special' item is added to the list to this effect.

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