Hello and welcome to my User Tip
You might be experiencing some performance issues with your Mac, spinning beach ball, just general slowness and you knew from your PC days about defragging computers.
Well that's for Windows because the way it writes files it breaks them all up, OS X doesn't do that on small files, so it eliminates the need for MOST users to require a regular defrag.
However in some cases people may wish to optimize and defrag their computers hard drives, it does make a difference however it's very risky. I outlined a safe way here that always gives you a bootable backup, plus it really optimizes the hard drive in the process.
First if your experiencing computer problems, backup your personal information to a storage drive first, then consult the fix it options as defragging won't necessarily solve problems in software.
So to begin with some facts:
1: Only hard drives require defragging. SSD's no need.
2: You don't need to regularly defrag a Mac's hard drive, OS X writes small files in one batch, eliminating a lot of the need to defrag a Mac regularly.
3: Hard drives are fastest at the beginning of the drive. and slowest at the very end. Data is first written on the top of the drive and works it's way down.
4: The first 50% of the drive is faster than the second 50% due to larger sectors and longer tracks which the heads have less to move and can gather more data at one time.
5: Over time a Mac can slow down as the adding and reduction of data, OS X upgrades and normal operation tends to move data making it less optimized.
6: If one wants to create a Bootcamp or second partition on the boot drive, there might be OS X data near the bottom where the second partition will go, it's sometimes not easy to move this data further up on the drive.
7: If there are bad or failing sectors on the drive, it can substantially slow down read speeds and the spinning beach ball effect occurs.
8: I don't advise using defragmentation software on a "live" system, it's dangerous and if it messes up your stuck. If anyone has defragged a PC before knows it's never really complete in actuality.
So my solution if your serious about performance, have large files like video that stretch across many sectors and want it in one piece, want to reduce bad sectors that can corrupt large and even small files, need to free up space for a partition or large file.
(Note: If you have Filevault enabled, forget it, I can't confirm if this will work or not with this procedure, consider not using it as it slows down a machine anyway and is currently cracked: How do I securely delete data from the machine? )
How to proceed
1: Reduce your boot drive user content so it's less than 50% of the drive filled (ideal) but not more than 80% filled. (not mandatory but preferred)
2: Use a blank powered external drive and a copy of Carbon Copy Cloner to clone the OS X boot partition (aka Macintosh HD) to the external drive.
3: The reboot the machine holding the option key down, you can boot from the clone.
4: Use Disk Utility to Secure Option Erase with the "Zero" or middle secure selection (10.7/10.8) the internal OS X boot partition, this will "Zero" out all the bits, and when it reads it back for confirmation, map off any bad sectors in the process, never to be written to again.
5: When completed, reverse clone. Run Disk Utility > Repair Permissions on both and all the OnyX (Macupdate.com) maintenance and cleaning aspects and reboot. OnyX cleans out the caches and lets them rebuild.
See #12 OnyX Routine here: Step by Step to fix your Mac
You will solve your Bootcamp partition formatting problem, your bad sectors problem, your defrag problems, corrupt or bloated caches and your optimization problems all in one batch. Also have a working bootable backup in the process which is loads safer than a live defrag.
Your Application's will be written to the "hot zone" the fastest part of the drive. They will load as fast as possible.
Your OS X system files will be written next, followed by your User account data last which tends to grow, expand and contact, suffer with more changes so the defragmentation that occurs will occur at the end and into the slow sections of the hard drive, not where Applications and System are.
Sure over time with OS X updates, upgrades, new app installs and such you can start seeing a loss in performance again (over many years), but another clone/reverse clone will solve that problem.
Also if you want to backup and defrag Windows Bootcamp partition (CCC doesn't do that), there is WinClone for that. I don't know how effective it is as I haven't used it.
There you go enjoy the new performance!
Note: SuperDuper is also cloning software, however I'm not as familair with it's operation, but am with CCC and how it write files for the optimization. (alphabetical order of the files on the root directory) Both softwares are payware, however I don't receieve any compensation for product or site mention.