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I've currently got a Mac Pro with data on one volume, and the OS/Applications sitting on an SSD. Both are backed up to a second partitioned drive as part of my overall backup strategy.

In both cases, the description of the scheduled task reads as follows:

If you have backed up to this destination in the past, only items that have been modified since the last backup task will be copied.

  1. Data Drive
    • Used space: 346.05GB
    • Backup partition: 442.67GB
  2. OS/Apps Drive (shows as bootable on startup)
    • Used space: 78.49GB
    • Backup partition: 76.13GB

I'll probably test drive a "let's use the backups and see what happens" test shortly, but does anyone have any idea as to what is causing the difference? If it helps any, the OS/Data split in the user files was done using this technique: http://eshop.macsales.com/Reviews/Framework.cfm?page=Tips/relocate3/relocate3.html

Is there a better backup technique to be using with this setup?

  • Strange that the OS backup is smaller than the existing OS. Unless you have stuff in the Trash that's using up 2GB. As for the data, that makes sense. It archives the data that has changed and adds the newer version, so I'd expect that to take up more space. – user3463 Sep 20 '11 at 6:31
  • Yeah, it was "different and smaller" that made me nervous about the OS clone. I hadn't thought of the Trash, so I emptied the trash, did a full restart, and re-ran the OS clone task. Now we're at OS = 78.47 GB and the backup is 76.12 GB, according to Disk Utility. – Fomite Sep 20 '11 at 6:47
  • This gets weirder. Running OmniDiskSweeper on both the OS drive and the Backup leaves the application claiming their size is only 0.4 GB different - 70.3 and 69.9 GB for the OS and backup respectively. The difference seems concentrated in 'private', and the 'var' directories, which are normally hidden. boggle – Fomite Sep 20 '11 at 7:00
  • I think it's time to test your backup to see if it's complete. – user3463 Sep 21 '11 at 4:04
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    @RandolphWest Well, I just ran two tests - booting under the assumption that the boot drive had failed, and then booting under the assumption both the boot and data drive failed. Besides being spectacularly slow going from an SSD to a 5400 RPM drive, things...seem to work. Applications are there, and work. My most important document remains intact. Not sure how to really do a full blown test outside using the backup for a while, which I'd rather not do, but whatever's different, it seems...not essential enough for getting back up and running. – Fomite Sep 21 '11 at 5:02
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From this site:

Dear Martin Weber,

Disk size calculations from an internal to an external disk are often different. I would not sweat the gigabyte differences as long as you can see all of the files on both disks. If Carbon Copy Cloner is properly configured, and if the backup ran successfully, then you should be all set even if the file count or disk space usage indicators differ. Still, it is definitely worth double checking.

– David Marx

  • The link above no longer works (for me at least). However, this question is (at least partially) addressed on the Carbon Copy Cloner web site at bombich.com/kb/ccc4/…. – Keith Bennett Aug 6 '15 at 14:17

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