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Here is Apple's official stance of defragging: most people don't need to do it. But if you routinely use large files that the OS doesn't automatically defrag on the fly, iDefrag can help.
 
Any file that is under 25MB will be automatically defrgged by OS X.

However, some people feel the need to defrag beyond that.
To do that, you need a 3rd party app, like iDefrag. I suggest making an excellent backup before embarking on that journey.
 
Interestingly, PC World (I'm a about to Switch person) did a test of before and after defrag test, and found that these days, defragging doesn't really do anything performance-wise. I can see how the Mac position (on the link provided earlier) is correct in this regard.
 
Interestingly, PC World (I'm a about to Switch person) did a test of before and after defrag test, and found that these days, defragging doesn't really do anything performance-wise. I can see how the Mac position (on the link provided earlier) is correct in this regard.

Quite true.

Defragging was great when disk speeds were low, so seek time made a difference. Now-a-days, with 7200RPM drives (or faster), that's become a rather moot point.
 
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