It is generally not necessary to de-fragment a Mac. Unless the hard drive is 80%-90% it is unlikely that any files will fragment. Also, de-fragmenting safer when hard drives were smaller and there was not as much data to move, de-frag, and then replace in real time.

The best way to De-frag is to save all of the data to another drive and then erase and reformat the drive.

1. Copy all information to another drive.
2. Double check that EVERYTHING is copied to a DIFFERENT drive.
3. Go to:Mac HD>Applications>Utilities>Disk Utility
4. Select the volume you wish to erase
(picture of the hard drive with the numbers as part of the name).
5. Click the Erase tab
6. Set the Volume format to "Mac OS extended (Journaled) and name the drive whatever you wish.
7. Take a deep breath
8. Click the erase button (not the tab at the top)
9. After the computer is done erasing; move all of the files that you temporarily saved to another drive back to your newly formatted drive.
10. Erase the temporary copy.