I have all my files & pictures (close to a terabyte) on an external drive. A few months ago I backed up this external drive to a 2nd external drive which took a few hours. To do so I dragged the icon of the first external drive on to the icon of the 2nd drive. Its been a few months and I would like to refresh this backup. If I drag the icon again on to the 2nd drive's icon will it just back up those files that have been changed, taking only a matter of minutes, or is it going to replace every file, taking hours?