Adding partition to disk creates unwanted third partition 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.
Use the following steps.


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


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


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.


Highlight the new partition by clicking on the pie piece. 


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


When finished, click on the "Done" button.


The final result is shown below.