If you have dual-booted your Mac computer with Windows using the Boot Camp Utility, you may find that you don't need as much space for the Windows partition as you thought. In the Mac OS X operating system, partitions and hard drive configurations are controlled through Disk Utility. Reducing the size of a Boot Camp partition takes only a few clicks with Disk Utility.
Click "Applications" on the Dock.
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Click "Utilities," then "Disk Utility."
Click your hard drive in the devices on the left. Click "Partition."
Click and drag the slider at the bottom of your "Macintosh HD" partition and drag it down to grant it more space or up to give it less space. The rest of the space will go to the Boot Camp partition.
Click "Apply" and wait for your computer to repartition these drives.