You do not need to defrag your Mac, especially with a freebie.
I run 8-10 Macs in a network for my photography business and have done so for years. Any notion that Macs or OSX do not need to be defragged is utterly wrong. HARD DRIVES begin fragmenting the moment you start using them. ALL hard drives, regardless of platform.
While it may be technically correct that OSX itself doesn't need to be defragged - perhaps - ALL hard drives need maintenance or severely fragmented drives will cause lots of frustrating spinning ball... Apple has always claimed OSX does not need to be defragged - a confusing comment - since it makes them a LOT of money since people often believe a slow Mac needs to be replaced when, in truth, Genius bar technicians often use Drive Genius 3 to defrag and optimize your dysfunctional Mac.
Hard drives become fragmented much more quickly if you move a lot of large files frequently, as opposed to lots of small files BUT small files can also add up, in time, to cause fragmentation. Once the drive is severely fragmented,k you get a LOT of spinnning ball on a Mac - period. A hard drive is like oil in a car engine - changing the oil now and then is mandatory - just like defragging is neccesary now and then if your Mac gets slow.
Example: In the office my business partner has been running an older iMac for 6 years. Just recently it progressed to being so sluggish - spinning ball constantly - that it barely ran at all. I admit to not having paid attention to her office Mac, so my fault... She was frustrated beyond belief - it was tough to accomplish any office task.
Yesterday I tested her drive for fragmentation using Drive Genius 3 to discover her drive was 25% fragmented - enormous for a Mac, in my experience. Naturally, my business partner felt she needed a new Mac. I wish I owned every Mac folks throw out due to nothing more than having a need for defragmentation...
Drive Genius 3 would <not> run on her 2007 Intel iMac 2 ghz machine so I resorted to an old copy of iDefrag which ran like a charm!!! This morning, after running iDefrag last night, her machine is as snappy as a brand new Mac!
Folks, do NOT believe those who say your Mac never needs to be defragged. Yes, Apple does say OS 10 doesn't need defragging BUT <your hard drive DOES need it> once it become severely fragmented. I am not a deep techie - I know only what keeps my Macs which often run 12 hours/day in good shape. I assure you Macs DO need to be defragged.
Be aware some utilities like TechTool Pro and Drive Genius do not work on all Macs. (call the utility tech support immediately if a newly purchased utility will not boot up on your Mac) In the case of my partner's iMac yesterday, only iDefrag would boot up but it did a great job!