Apple's Boot Camp utility is a software assistant that lets you install Windows on Macs that have Intel Processors. Once you've installed Boot Camp, you will be able to run both operating systems at once and seamlessly switch from screen to screen between them. To install Windows, Boot Camp requires that you partition your hard drive. This process divides your hard drive into two virtual drives, one for Mac OS and one for Windows. Unlike older Macs, Intel-based Macs running recent versions of Mac OS X let users change the size of the Boot Camp partition after creating it.
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Install a recent version of Mac OS if necessary. Resizing existing partitions requires Mac OS 10.4.6 or later. Most Intel-based Macs shipped with this or a later version already installed, so this step is unlikely to be necessary for most users.
Back up your drive by copying important files to another drive. This is always a good idea before doing anything to your main hard drive.
Open the Mac OS Terminal by double-clicking on "Terminal" in your "Utilities" folder. This gives you access to the UNIX commands that run underneath the graphical interface of a Mac.
Find your partition by typing "diskutil list" and pressing "Enter." You will see a list of all partitions on the hard drive. The boot disk is "/dev/disk0," while others will be listed as "disk1," "disk2," and so on, each with an identifier header that tells you the name of that partition. Look for the one that matches the name of your Boot Camp Drive.
Discover the possible size for your partition by typing in:
diskutil resizeVolume disk_identifier limits
Replace "disk_identifier" with the name of your Boot Camp partition and hit "Enter" to view a list of the current, minimum, and maximum sizes for the partition.
Change the partition size by typing "diskutil resizeVolume" followed by the identifier for your Boot Camp Partition and the size you want it to be. This size is written in gigabytes, but the command drops the "B" to become simply "G."
For example, to resize a partition entitled "BootCamp" to 100 gigabytes, the command is:
"diskutil resizeVolume BootCamp 100G"
Once you have typed your command, hit "Enter" to resize the partition.