If you have installed Windows on your Mac computer, then you will have initially partitioned your hard drive with the Boot Camp Assistant. If you have discovered that you want to give more space to your Windows partition than you initially did, then you can resize the partition. Using the Mac application "Disk Utility," you can resize partitions on the fly. This is a simple process.
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Click the "Applications" folder on the Dock. Click "Utilities," then "Disk Utility."
Click "Partition." You will see your hard drive split between the two Boot Camp partitions.
Click and drag the middle slider in between each partition. Shrink the "Macintosh HD" partition to give your "Windows" partition more space.
Click "Apply" to save changes.