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What to do when About This Mac lies to you

It may be perplexing to see the system report you have zero movies, music, and photos on a relatively populated hard drive. Here is how to get the facts.

If you are running OS X Lion or Mountain Lion and need to see some quick information about your Mac's hardware, you can do so by selecting "About This Mac" from the Apple menu, and then clicking the "More Information" button to launch Apple's System Information tool. This tool can be used to generate a detailed system report, but by default will show a brief summary window that includes a breakdown of storage usage.

Storage information calculation error in OS X
When this error occurs, you may see one or more categories show incorrect size calculations. Screenshot by Topher Kessler/CNET

The storage view shows your hard drive and includes a small diagram that breaks down the amount of space used by audio files, movies, photos, applications, backups, and then any other file type. However, it can sometimes happen that you see numbers in this window that are obviously incorrect, and About This Mac may even report 0KB used for a file type when you know far more than this is on your system.

For example, on my Mac I have tens of gigabytes of audio files, movies, photos, and applications installed; however, the About This Mac window reports that the system contains 0KB for each of these categories.

If you see this on your Mac you might quite logically be concerned about the integrity of your data, but this issue is simply a result of an incorrect indexing of your hard drive and does not reflect the true contents of your system. In order for the system information tool to properly report the space usage on the drive, it needs to have a Spotlight index that is both properly built and enabled.

  1. Make sure Spotlight is on
    To address this problem, first make sure Spotlight is enabled for your system. To do this, open the Terminal and run the following command:

    mdutil -s /

    If the result of this command shows "Indexing disabled" then enable Spotlight by running the following command:

    sudo mdutil -i on /

  2. Hard drive index progress
    During indexing, the drive space used will continuously update. Here the system progressively identifies files as it indexes the drive. Screenshot by Topher Kessler/CNET
  3. Reindex the hard drive
    The next step is to have the system reindex the hard drive, which can be done in two ways. The first is to open the Spotlight system preferences and then drag your hard drive to the Privacy list, then back off again. This action will clear the index and cause the system to rebuild it from scratch. The second approach is to run the following Terminal command, which will likewise erase the Spotlight index:

    sudo mdutil -E /

    After the index has been erased, you will see the indexing progress bar appear in the Spotlight menu. Reindexing may take a while to complete (sometimes several hours depending on the type and size of the drive's contents), but after it's done, the system information utility should properly report file sizes.

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