Partition a Hard Drive in Mac OS X

Apr 26, 2011 - 27 Comments

How to partition a Mac hard disk drive If you want to create a new partition, modify a partition table, or remove an existing partition from any hard disk drive in Mac OS X, you won’t need to use anything fancy other than the bundled Disk Utility app that comes with all versions of OS X. Disk Utility has all the tools on hand necessary for modifying partition tables of any Mac drive, and it’s easy to use after a little guidance.

Before going any further, be sure to have a full backup of your hard drive and all important data and documents before adjusting partitions in any way. This is to insure file recovery is simple just in case something goes wrong with the partitioning process, the simplest way to do this is to start a quick manual backup through Time Machine and letting it complete. Once you have an adequate backup made of the Mac, proceed with this walkthrough to learn how to add a new partition, modify and resize existing partitions, and how to remove them too.

How to Add a New Hard Drive Partition in Mac OS X

  1. Launch Disk Utility from /Applications/Utilities/
  2. Select the hard disk you want to partition from the left side of the app
  3. Click on the “Partition” tab
  4. Click on the + button to add a new partition
  5. Specify a name for the new partition, select a filesystem type (Mac OS Extended Journaled is default), and choose a size either by manually entering a capacity or by dragging the slider bar in the partition map
  6. Click on “Apply” to create the new partition

Partition a Mac Hard Drive

You can make partitions any size as long as you have the available disk space to accommodate it, and creating the partition shouldn’t affect your existing filesystem either as long as there is free space. Nonetheless, there is always a chance something could go wrong, which is why I recommended you backup your drive first.

After you have clicked ‘Apply’ to create the new partition, it will be immediately accessible in the Finder to use how you want. A new partition will behave like a new hard drive, and it will appear on your Desktop as a new drive which can be ejected, mounted, formatted, just like a hard disk.

I partitioned my drive before installing Mac OS X Lion so that I could maintain my stable Mac OS X 10.6 system software while still exploring the Lion 10.7 Developer Preview. Another common use is to partition large external hard drives for a specific Time Machine backup partition, and then a separate storage partition. Time Machine will backup a drive until the available space is taken, so if you set it to backup to a partition, it will only take up that space and leave the other partition alone, allowing the drive to serve multiple uses and allowing you to effectively share a single hard drive for both Time Machine and other uses.

Deleting a Partition

Removing partitions is just as easy as creating one. All you need to do is follow the steps above to get to the partition table, select the partition you want to delete, and then click the “-” button rather than the plus icon. Remember, if you delete a partition, you will lose data that exists on it. Click “Apply” for changes to take effect on the drive.

Resizing Existing Partitions

Resizing an existing partition to a new size allows to either grow or shrink the total capacity available to a partition. This can be done through Disk Utility very easily by doing the following actions. As usual, backup the drive before beginning to be safe:

  1. From the “Partitions” tab, simply drag the separating bar between existing partitions either up or down to resize as needed
  2. Alternatively, click the partition to resize, then enter a new size value in GB in the Size box that is alongside the partition map
  3. Choose “Apply” to resize the partition

You do not need to reboot for any changes to take effect. As always, have a backup ready before making any changes to partitions, it’s rare that something will go wrong but in the event it does, you’ll be happy to have a backup handy so you can restore as quickly as possible.

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Posted by: Paul Horowitz in Mac OS X, Tips & Tricks

27 Comments

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  1. Darian says:

    You need to defrag your HD before adding a new partition, especially when the HD has been in use for a while.

    • Stephen says:

      Not totally correct. If you have a Mac Journaled file system (as is shown in the screen-shots), there is no such thing as defrag.

      • Darian says:

        Not sure you’re right about that. I had this situation a month ago. My HD (with no partitions) was/is Mac Journaled. The only way I could partition it was by first defragging. Until I tried that, I constantly got failures.

        • Lance says:

          Most defrag processes remove VM and sleepimage files, so they create more space, thus allowing that couple of additional GB needed to create the new partition. If you make the new capacity for the startup disk too tight, the system could get slower later, since it won’t have the extra space needed for VM and sleepimage files. It’s better not to defrag when partitioning fails. Instead, make the new partition smaller. The real problem is having a small capacity drive and wanting to pretend it is larger.

    • Pierre says:

      Mac OS X doesn’t need to be defraged. Plus if you own an Solid State Drive (SSD) the manufacturers highly recommend you DON’T defrag as this seriously reduces disk life.

      Read Apple’s support page: http://support.apple.com/kb/ht1375

  2. Joseph says:

    Gah why didn’t I think to partition my backup drive! It’s a hassle for me to delete them now I can just reformat the partition!

  3. Land Of Tech says:

    Whats the chance of screwing up your hard drive? i want to try Lion but don’t want to risk my data and lose it for the second time (first time mac got stolen).

    • Jeromy says:

      “Whats the change of screwing up your hard drive?”

      Repartitioning is one of the highest risk things you can attempt with your data. If something goes wrong, like a power failure or human error, the damage to your data will be catastrophic.

      So don’t do it without backing first. At least buy a cheap external USB harddisk.

  4. Filip Wahler says:

    I was just reinstalling 10.6 and repartitioned my harddrive. It’s important to click ‘Options…’ and select ‘GUID Partition Table’ before you apply the new volume scheme.

  5. Pierre says:

    When I setup a Linux system it always requires that there be a partition dedicated to SWAP in the tables. I can’t find anything about this for the Mac, does this mean the Mac doesn’t use a swap partition? Or the utility takes care of all these little details automatically when you proceed with an installation?

  6. David says:

    For some reason the + button to add a new partition is grayed out. What am i doing wrong or missing?

    heres my screenshot
    http://a.yfrog.com/img739/6183/fdb.png

    • M says:

      You’re running Snow Leopard which treats the boot drive differently, you may need to boot off a DVD or USB key to partition the primary disk in that case. In OS X Lion, you can partition from the boot drive.

      • Lance says:

        Notice the “Size” is 499.76GB and the “Available space” is also. Time to try “Repair Disk”. Something is wrong with this picture. Disk Utility calls it the startup disk, yet it has no files.

  7. Benzamin watson says:

    I really appreciate this valuable information and want to share my own experience with you. I am using third party tool for partitioning of hard drive with some advanced features of hide/reveal and automatic adjustment of free space together in your drive is possible only with stellar partition manager.

  8. [...] Optionally, click the [+] icon if you need to add a partition, read more about that here [...]

  9. Fais says:

    I cant do this since i have one HD ? help pls

  10. Gabriel Kemikal says:

    My friend helped me to create a partition on my HDD and now I want to remove it. The partition is only 50 gigs and was meant to be a simple backup partition, only somehow everything I do ends up in the partition and not on my startup disk. I’m copying over all my files to my main HDD now and then deleting everything from the partition; though whenever I try to do ANY action to the partition, I am told that the partition cannot be modified.

    Can anyone help me?

    • Lance says:

      Check the format. Is is NTFS? Then ask your friend why he did that. Mac OS doesn’t come with the option to erase files on NTFS. You don’t need to erase the volume to remove the partition. Just remove it it.

  11. Cory says:

    When trying to create a partition on my brand new Mac mini 2012 with fusion drive, it failed and left my harddrive split into two partitions, one being free space. I can’t change the free space to another filing system or delete the free space. I also can’t extend my first partition with mountain lion on it. Now I’m stuck, when I verify disk it says to repair disk. I tried repairing the disk and it said to repair the disk…even though I was trying to. Same response in recovery mode. Does anyone have any ideas?

    • Lance says:

      If “Repair Disk” finds errors, repair again. If it is unable to repair, you might jump through a lot of expensive hoops to use a third-party repair utility, or just erase and reinstall.

  12. Daizy says:

    Follow these instructions to remove your Mac partition:-
    Shut down your Mac
    Start up holding down Command ⌘ + S. (This will boot you into Single User Mode.)
    At the prompt, type in the command fsck -fy.
    When that finishes, type reboot.
    Try resizing again in Disk Utility.

    • Lance says:

      You can “Repair Disk” in Disk Utility without booting to single-user mode. fsck is just a simpler form of disk repair. It isn’t better than Disk Utility.

  13. [...] basic process may be familiar to Mac users who have setup drive partitioning and backups before, but we’ll cover every step to be sure everything is configured [...]

  14. [...] ran into this twice recently, first when attempting to modify partitions on a drive, which came right along with a separate “partition failed” error, and again [...]

  15. [...] for booting from external volumes, whether it’s to troubleshoot problems, repair disks, partition, format everything, update, or even reinstall OS X. We’ll cover the two easiest ways to do [...]

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