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How to Defragment Mac OS X

written by: Michael Dougherty•edited by: Lamar Stonecypher•updated: 1/17/2010

Defragmenting a Mac’s hard drive is not necessary all the time because Mac OS X uses a journaled file system that automatically handles defragmentation on the fly. But you still need to run a third-party defragmenting program from time to time to check over all the files on your hard drive.

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    What Is File Fragmentation?

    File fragmentation occurs when parts of a single file are scattered around many different places on a hard drive. This usually happens when files are created, deleted or modified. For example, suppose you create four new files then delete the third one. If you create a fifth file that is larger than the third file, the OS X file system will put a part of the fifth file in the space occupied by the third file and the remainder after the fourth file. When you will access the fifth file, the OS X will have to look in two different places, resulting in slower access to the file.

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    Do You Need To Defrag?

    The Mac file system does not need to be frequently defragmented because it is designed in such a way that it works with some degree of fragmentation. Slight fragmentation does occur but it does not affect a system’s performance much. But due to the nature of work you’re doing, the nature of your files and the order in which the files are sliced can all have an affect on file system’s performance. Generally, you will not notice significant degradation in performance but if you frequently create and delete files, your hard drive may become a little defragmented to the level that you will notice a slight slow-down of your OS X performance.

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    How To Defrag Your OS X?

    If you feel that your OS X has slowed down, you can use defragmenting software for OS X. Apple does not provide any defragmentinidefrag g utility with OS X, but you can search for available Mac OS X defrag software in the Macintosh Products Guide . I personally use iDefrag, a very nice Mac OS X defrag utility. It is easy-to-use and comes with plenty of options. iDefrag supports HFS+, case sensitive filenames, and journaling. The demo version of iDefrag is free to use, the full version of costs $37.

    You should plan to run your defragging software during a time when you'll not need to use your Mac. It can take several hours for any software to defrag a Mac hard drive; therefore I recommend you run the program overnight.


    Defragmenting a hard drive requires a large amount of data to be rearranged. When you run the defragging software, there is a very slight risk of losing specific data if the software fails in recovering the data while the portion of it is in transit. So, you should keep a backup of your hard drive before running defragmentation software.

    Once the defragmentation completes, notice whether your hard drive improved the Mac’s performance. Running applications and opening large files should be faster by a few seconds. When more than 75% of disk is full and then defragmented, the performance increase will be more noticeable.