<p>Carbonite, Mozy, Time Machine, and any other decent backup app will only backup changed or new files - whatever’s changed since the last backup. The very first backup will of course backup the entire directories that the user configures to be backed up (which might include the whole disk).</p>
<p>There’s no reason at all to backup what’s already backed up when nothing’s changed and it would only busy-up the system and extend the time to transport all the data.</p>