How to Partition a Drive in OS X

Partitioning a Drive

When you partition a drive, you essentially make your computer see two different drives where there used to be one. This is useful for a variety of reasons and is something you may find yourself needing to do, but the process on a Mac is a bit complicated. It may be easy to do visually, but using Voiceover, there are some tricks you should know. So, let's get started.

First, open Disk Utility. If you want to browse to it manually, it is in applications/utilities, but I find it easiest just to hit command-space, start typing the name, and press enter when I hear Disk utility spoken.

Once opened, you should be on a table of disks and volumes. Find the disk you want to partition in the table. Note that a volume is not the same as a disk; disks can hold multiple volumes, though generally each disk will have only one volume. Once you partition your disk, it will have two.

For the sake of this article, I will assume you want to add a second partition to your main hard drive. You should already have a volume called Macintosh HD, and that is on a disk with a name along the lines of "Hitachi [model number]". Again, since you want to partition the drive, choose the Hitachi (or whatever your disk is called) in the table and not the Macintosh HD volume.

Now find the five options, and activate the Partition button. If you don't see it in the options, you have chosen a volume in the table and not a disk. Once activated, you will see all the partitions on this drive; again, unless you are a Bootcamp user, this should be only Macintosh HD.

Now the fun part: adding your new partition. First, find the text field that displays the size of Macintosh HD, which should be just under the total capacity of your drive. For instance, my Mac Mini has a 500gb hard drive, and my Macintosh HD volume is 499.25gb. You must first change this to be smaller, freeing up room for your new partition. Since I don't need a lot of room on my new partition, I gave it 50gb. I did this by erasing what was in that "size" text field, and replacing it with 449 instead. I've now told my Macintosh HD partition to be 449gb, and i have 50gb of unused space to play with.

Next, now that my Hitachi drive has that 50gb free, I can add my new partition. yes, the 50gb was free (as in not used) before, but it was assigned to the Macintosh HD volume. By shrinking that volume I have now made the 50gb free to be re-assigned. To start that re-assignment, go to the "add" button and activate it.

Almost done. VO-left until you find a scroll area; interact with it, and you will see the partitions (by default, your newly added partition is named Macintosh HD 2, or a similar version of whatever the previous partition on the drive is called). Find it in the scroll area and vo-space on it. It is not identified as a button or anything else that responds to VO-space, and you will get no feedback when you issue the command, but this will cause the window to ask for information on the new partition, instead of the old one. Stop interacting with the scroll area, and fill out all the details you want - name, size, file system, and so on.

Once you are happy with all your settings, it's time to make the changes. Up until now, you've just been setting everything up, but nothing has been done to your hard drive. Find and activate the "Apply" button to start the process, then respond to the confirmation that will appear. Once you do, your hard drive will be partitioned according to the settings you made. Once complete, you will be back in the window you started from, only now, both partitions will be listed.


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Submitted by Ivor123 on Friday, July 31, 2015

Alex, Thanks for your excellent guide. I'm hoping you can help me, because my attempt to partition my drive isn't working quite as described.

After I select my Macintosh HD, I have two boxes that list the size. When I try and edit the size in the first box, I get a dialogue that has the word "null" in it. I can either cancel or remove. I can't do anything with the second box that lists size. Do you know anything about this problem? Thanks in advance for your assistance.