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Disk Utility 12.x: Partition a disk

Partition a disk using Disk Utility

Partitioning a disk erases all the data on it, assigns it a partition scheme, and then divides it into sections, called “partitions,” which appear on the desktop as disks. You may need to partition a disk if you want to install multiple operating systems on the disk or if you need to change the disk’s partition scheme. (For example, if a disk was formatted to start up a PowerPC-based Mac computer, you need to change its partition scheme to use it to start up an Intel-based Mac.) You can also create partitions to help organize your files.

If you’re partitioning your disk because you want to install Windows, use Boot Camp Assistant, in the Utilities folder in Launchpad.

Partition a startup disk

If you want to partition a disk that you’ll use to start up your Mac, follow these steps.

Partitioning a disk erases all the files on the disk. Be sure to copy any files you want to keep to another disk before you partition the disk.

You can specify the number of partitions and the size of each one. If you want to use one partition as your Mac OS X startup disk, be sure it’s large enough to install the Mac OS X software and any applications and files you’ll use.

  1. If you’re partitioning an external disk, make sure it’s connected to your computer, and then go to step 5.

    If you’re partitioning the disk you used to start up your Mac, print this page so you can refer to it later. To print this page, choose Print from the Action pop-up menu (looks like a gear).

  2. Choose Apple menu > Restart. Hold down the Command (⌘) and R keys as your computer restarts.

    When you see a white screen with an Apple logo in the middle, you can release the keys.

  3. In the Mac OS X Utilities window, select Disk Utility, and then click Continue.
  4. Select the disk that you want to partition and click Partition.
  5. Choose the number of partitions from the Volume Scheme pop-up menu.
  6. Click each partition and type a name for it, choose a format, and type a size. You can also drag the divider between the partitions to change their sizes. If a partition’s name has an asterisk beside it, it’s shown larger than its actual size in order to display its name clearly.
  7. If you’ll be using a partition as a Mac OS X startup disk, click Options, and choose the the GUID partition scheme.
  8. Click Apply.

After you partition a disk, a disk icon for each partition appears in Disk Utility, and in the sidebar of Finder windows.

Partition a non-startup disk

If you want to partition a disk that you won’t use to start up your Mac, follow these steps.

Partitioning a disk erases all the files on the disk. Be sure to copy any files you want to keep to another disk before you partition the disk.

  1. If you’re partitioning an external disk, make sure it’s connected to your computer.
  2. Open Disk Utility, in the Utilities folder in Launchpad.
  3. Select the disk that you want to partition and click Partition.
  4. Choose the number of partitions from the Volume Scheme pop-up menu.

    If you can’t choose the number of partitions, follow the steps for partitioning a startup disk.

  5. Click each partition and type a name for it, choose a format, and type a size. You can also drag the divider between the partitions to change their sizes. If a partition’s name has an asterisk beside it, it’s shown larger than its actual size in order to display its name clearly.
  6. Click Options and choose one of the following:
    • If you’ll be using the disk on a Windows computer, choose the Master Boot Record partition scheme (also known as “fdisk”).

    • If you’ll be using the disk only with Mac computers, choose GUID Partition Table.

  7. Click Apply.

After you partition a disk, a disk icon for each partition appears in Disk Utility, and in the sidebar of Finder windows.

Create new partitions on a disk

You may be able to create new partitions on a disk without losing any of the files on the disk. Each partition works like a separate disk. You may need to create a new partition if you want to install multiple operating systems on your computer. You can also create partitions to help organize your files.

As a precaution, back up your data before creating new partitions on your disk

  1. Open Disk Utility, in the Utilities folder in Launchpad.
  2. Select the disk where you want to create a new partition and click Partition.
  3. Select an existing partition in the Volume Scheme list, and click Add (+).

    Disk Utility splits the partition into two, leaving the data from that partition in one of the new partitions. If the partition is less than half-full, Disk Utility creates two partitions of equal size. Otherwise, it creates one partition large enough for the data, and another partition with the remaining space.

  4. Resize the partitions as needed.

    You can drag the dividers between the partitions in the Volume Scheme list, or you can select a partition in the Volume Scheme list, and then enter a value in the Size field.

  5. Choose how to format the partitions that will be erased or created.

    For each new partition, select it, enter a name, and then choose a format.

  6. Click Apply.

Enlarge a partition

If you have multiple partitions on a disk, and one is running out of space, you may be able to enlarge it without losing any of the files on it.

To enlarge a partition, you must delete the partition that comes after it on the disk and then move the partition’s end point into the freed space. You can’t enlarge the last partition on a disk.

All data on the deleted partition will be erased. Be sure to back up your disk’s data before you begin.

  1. Open Disk Utility, in the Utilities folder in Launchpad.
  2. Select the disk that contains the partition you want to enlarge, and then click Partition.
  3. In the Volume Scheme list, select the partition beneath the partition you want to enlarge, and then click Delete (-).

    The partition is immediately erased, leaving the space it formerly occupied available.

  4. Resize the partition.

    You can drag the divider at the bottom of the partition in the Volume Scheme list, or you can select the partition and then enter a value in the Size field.

  5. If the enlarged partition doesn’t use all the space from the deleted partition, you can create new partitions in the unused space.

    To create a new partition, click Add (+), select the new partition, and choose its name, format, and size.

  6. Click Apply.

    Disk Utility enlarges the partition and creates any new partitions.

Last Modified: Jul 11, 2012
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  • Last Modified: Jul 11, 2012
  • Article: PH5845
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