There are two types of fragmentation that can occur on a hard drive:
This is a natural occurrence where the contiguous free space of a new drive becomes separated over time. As files are created, written, and deleted on the hard drive, there is less consecutive free space to write a complete file. The once large block of free space becomes fragmented into many smaller blocks of free space. This occurrence leads to data fragmentation because files have to be broken into multiple pieces to fit into the smaller free blocks. As the drive use nears capacity, both types of fragmentation become more pronounced.
As mentioned, data fragmentation occurs on a hard drive when there is not a block of free space large enough to contain the complete file. This causes the data to be split into separate pieces as it is written to the hard drive; which causes slower access time. When accessing the file, the hard drive has to search for each individual piece of data instead of accessing one large contiguous chunk. This affects all files, including system files and application files.
Mac OS X Defragmentation:
Mac OS X automatically minimizes the potential for fragmentation by writing new data to the largest available free space block; however, this optimization is dependent on external factors such as outstanding writes to the disk. Files can still be fragmented when they are opened multiple times even if a large enough block of free space exists to contain the complete file.
Mac OS X will automatically defragment files that meet the following conditions:
Drive Genius 3 Defragmentation: Drive Genius 3 addresses both free space and data fragmentation on a hard drive. The Defrag feature in Drive Genius 3 reorganizes the fragmented pieces of all files, no matter what their size or fragmentation level, and compacts their content into one contiguous block for optimal access times. In addition, Drive Genius reorganizes the files themselves so they exist in one contiguous block. By compacting the files, the free space is returned to one large block for the Operating System to write new files without data fragmentation.
Defragmenting your drives will lead to faster application launch, and faster access to your large media files, databases, and virtual machine images.