Disk fragmentation is an old problem that has affected every operating system throughout history. File fragmentation occurs when a single file isn’t located in the same physical location on the disk, but is scattered around. OS X does a great job at minimizing file fragmentation by rewriting files in contiguous space when a file is opened, is under 20MB and contains more than eight fragments. This works quite well to prevent heavy file fragmentation, but what it doesn’t prevent is free space fragmentation.

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