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I'm using disk utility to try to divide the partition I'm working on, which is 51.28 GB, into one main partition (to run OS X on) and one smaller HFS+ partition (on which I will eventually install GRUB to boot Arch Linux, which is in other partitions). However, using Disk Utility, when I have this partition selected and click the + button, whatever size partition I try to create, Disk Utility shrinks the current partition to 25.6 GB, creates a partition of the size I select, and then creates a third partition out of the remaining space. Does anybody know why I might be experiencing this problem? I am running OS X version 10.11.5 on a early 2015 13" Retina MacBook Pro.

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Use the following steps.

  1. Open the Disk Utility application. Highlight the appropriate drive. Below is an example.

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  2. Click the the icon labeled Partition. The following popup pane will appear.

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  3. Determine the size you want for the new partition. Subtract this value from the size of the current partition. In this example, I need a 30 GB partition, so the difference will be 69.656 GB. Replace the current "Size:" value with this difference, then press the return key. Below is the result.

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  4. Highlight the new partition by clicking on the pie piece.

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  5. First, select the "Format:", then enter a name in the "Partition:" field. Finally, click on the "Apply" button.

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  6. When finished, click on the "Done" button.

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  7. The final result is shown below.

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  • This doesn't seem to be working for me: I get almost to the end, but not quite. Screenshots at this link. – Filanator May 23 '16 at 3:31
  • If you have already preformed an partition operation with the Disk Utility application, you may have to unmount the drive and remount before applying the next change. Since this is your boot drive, which can not be unmounted, you will have to restart the computer. – David Anderson May 23 '16 at 4:14
  • For what you are trying to do, the Disk Utility application can be a useful tool, but you will probably will need to use a few Terminal application commands. If I were you, I probably would have used the Disk Utility to shrink my OS X volume and create a new volume. The instructions above explain how to do this. But after that, I would have booted to Internet Recovery and used the gpt command to finish the job. – David Anderson May 23 '16 at 4:28
  • Have restarted, but the same thing is happening. – Filanator May 23 '16 at 4:31

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