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HT3275: Time Machine: Troubleshooting backup issues

Learn about Time Machine: Troubleshooting backup issues
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HT3275 How Does Time Machine Work?

6553 Views 3 Replies Latest reply: Nov 20, 2012 1:13 PM by LaPastenague RSS
DThenriksen Calculating status...
Currently Being Moderated
Nov 11, 2012 2:23 AM



I've been interrested to know how time machine works. I only back up to my external hard drive (Seagate 2TB) every month but even if i have barely used it, it backs up over 100GB of data puzzels me.


Any help would be exellent!



Time Capsule, Other OS
  • mende1 Level 10 Level 10 (89,470 points)
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    Nov 11, 2012 2:26 AM (in response to DThenriksen)

    Welcome to the Apple Support Communities


    Time Machine makes a full backup the first time you connect the Time Machine disk, and it makes acumulative hourly backups. Then, it deletes the hourly backups and Time Machine saves a backup per day.

  • TimCullis Level 1 Level 1 (0 points)
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    Nov 20, 2012 7:29 AM (in response to DThenriksen)

    Yes, that's what it says on Time Machine preferences, and the Apple support document isn't very helpful when it comes to trying to understand what's going on.


    Why is it backing up 100GB at a time?  Once a month isn't very often for a backup and it's likely that during this time you've updated some large fileset, maybe a photo library, music, etc.  I'm using hourly backups and these are often several GB.  But because the Time Capsule is on the network I often don't notice the backup happening even when I'm working on the system.


    I just tried this.  I backed up, then went into iTunes and deleted just one song (from a 30,000-song library), then backed up again.  The backup was 14MB which was probably the iTunes index file.  Then I went into one of my iPhoto libraries, deleted one photo and backed up again and this time the backup was much smaller, just 500MB.  Also I find that sometimes it says it is going to back up so much data, then actually backs up less.


    Only files that have changed are backed up, and hard links are made to other files, so each backup session appears to the user as if it is a full backup rather than an incremental backup.


    The wikipedia entry gives a better explanation of how Time Machine works that Apple's site (LOL), see

  • LaPastenague Level 7 Level 7 (32,640 points)
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    Nov 20, 2012 1:13 PM (in response to DThenriksen)

    Another excellent reference from local hero, Pondini.


    He has extensive info.



    How it works at the top there. Loads of gory details.


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