Hey everyone, This isn't a question, just some anecdotal evidence supporting defragging HDs capable of booting OS X. Might be an interesting read or helpful for someone in the future. My computer is a >3 yr old 2.5 GHz C2D MBP (early 2008). It came with Leopard pre-installed. I installed WinXP via Boot Camp and linked the Boot Camp partition to Parallels (v3 or 4 at the time). I upgraded to Snow Leopard around 10.6.2, then upgraded XP to Win7 and Parallels to the most recent version. In short, lots of changes to partition map and HD. I was also starting to reach full HD capacity -- 200 GB used with 60 GB free. Yesterday I decided I want to clean install Lion by making a new partition, installing Lion fresh, and gradually moving files from the Snow Leopard disk to new Lion one. I erased Boot Camp (with Boot Camp Assistant) to free up space. I then tried partitioning with Disk Utility and within Terminal. But unfortunately it failed every time (always distressing on OS X, where stuff usually just works). I kept getting the following message from Disk Utility: "Error: -9899 The partition cannot be resized. Try reducing the amount of change in the size of the partition". I tried resizing to as small as 10 GB but no luck. I then erased Parallels using the official instructions, thinking maybe some part of the HD is being secretly allocated to Parallels. No help. I tried doing all the typical Disk Utility maintenance (verify + repair permissions + disk). Still no dice. I eventually opened Console.app to see what's going on, and I noticed the following message from the kernel: hfs_reclaimspace: Error reclaiming datafork blocks of fileid=23184815 (error=-32735). I tried searching for and finding this 23184815 file with Terminal, but no luck (most of the folders told me I had no permission to look inside, maybe I forgot to sudo?). I reasoned that there is some fragmented data that is breaking the contiguous free space required for a new Partition. I'm not savy with Terminal or UNIX so I went ahead and purchased iDefrag 2.0.5 for $30 and defragged my disk after somehow booting into the program. Process took ~8 hr! I reclaimed 20 GB and the system runs sooo much smoother. No more beach balls! Even animations are faster. Before I had to wait at least 1 min after booting for the system to become responsive (granted, I've installed a lot of **** over the years like CoverSutra, TotalFinder, Google's Music Beta uploader, iStat Menus, GeekTool, etc.). And, most importantly, partitioning worked beautifully and Lion installed without any problems. In summary, defragging is good for older and/or near capacity HDs that are capable of booting into OS X. Anyone who says that defragging is not necessary or is done automatically on Macs is full of ****. Strongly consider defragging if your disk is slowing down and you are running low on HD space. You could always simply nuke the HD and reinstall OS X fresh, but that's like performing neurosurgery for a simple headache... or getting gastric bypass to lose a couple of pounds... Just make sure to backup your HD before you mess with it!