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Do I need to optimize?
You probably won't need to optimize at all if you use Mac OS X. Here's why:
Hard disk capacity is generally much greater now than a few years ago. With more free space available, the file system doesn't need to fill up every "nook and cranny." Mac OS Extended formatting (HFS Plus) avoids reusing space from deleted files as much as possible, to avoid prematurely filling small areas of recently-freed space.
Mac OS X 10.2 and later includes delayed allocation for Mac OS X Extended-formatted volumes. This allows a number of small allocations to be combined into a single large allocation in one area of the disk.
Fragmentation was often caused by continually appending data to existing files, especially with resource forks. With faster hard drives and better caching, as well as the new application packaging format, many applications simply rewrite the entire file each time. Mac OS X 10.3 Panther can also automatically defragment such slow-growing files. This process is sometimes known as "Hot-File-Adaptive-Clustering."
Aggressive read-ahead and write-behind caching means that minor fragmentation has less effect on perceived system performance.
For these reasons, there is little benefit to defragmenting.
Mac OS X systems have hundreds of thousands of small files, most of which are rarely accessed. Optimizing them is a major effort for very little practical gain. There is also the a chance that one of the files placed in the "hot band" for rapid reads during system startup might be moved during defragmentation, which would actually decrease performance.
If you think you might need to defragment, try restarting first.
If your disks are almost full, and you often modify or create large files (such as editing video, but see the Tip below if you use iMovie and Mac OS X Panther), there's a chance they could be fragmented. In this case, you might benefit from defragmentation, which might be performed with third-party disk utilities. Another option is to back up your important files, erase the hard disk, then reinstall Mac OS X and your backed up files.
