"OS X does not need to defrag, the previous poster clearly is not aware of this. Defragging a HD is something from the 1990s and before, it has not been necessary even on Windows machines for at least 7-8 years!"
I am aware of the claim that "OS X does not need to defrag". That is only a half-truth.
The Mac OS -will- re-concactenate -some- files of certain sizes, but not all of them.
More importantly, it is not just the "files" on a drive which can become fragmented.
It is the FREE SPACE -between- the files that grows fragmented over time.
Look at an older drive with a defragging utility, and you will clearly see this.
The problem with such "fragmented free space" on older HDD's is that the OS has to "go looking" for space as it writes temp files, maintains swap files, etc.
When a platter-based drive is defragged, all the free space is re-concactenated "behind" the files, into one contiguous space. No more waiting as the drive (and OS) have to sort through literally thousands of "free space fragments" to find a location large enough to handle disk writes.
One doesn't need a defragger to re-concactenate a drive.
This can be done by:
1. Cloning the drive to a backup drive using CarbonCopyCloner or SuperDuper
2. Booting from the backup
3. Re-initializing the source drive
4. RE-cloning the contents of the backup BACK TO the source drive.
This will re-group all the files together at the head end of the drive, with all the free space behind it...