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OS X: Installing OS X on an external volume

Some disk partition configurations may result in the OS X installer reporting that it could not create a Recovery System. If this happens, and you are unable to install or upgrade OS X, you may need to backup and erase your hard disk first. You can use Internet Recovery, or Recovery Disk Assistant to do this. You can also install OS X on an external volume.  

Installing OS X on an external storage device

Use these steps to install OS X from your Mac to an external USB, FireWire, SDHC or SDXC card, or a Thunderbolt storage device.

Note: Normally you should not need to erase your drive to install, upgrade, or reinstall OS X. These steps are designed to help you create an external volume that includes a Recovery system. 

Important: These steps erase the storage device. A storage device that is already formatted to support Windows (formatted for FAT or NTFS file systems) will be reformatted. You should back up any important files that are on the device to a different drive.

  1. Attach a USB, FireWire, or Thunderbolt storage device to your computer, or insert an SDHC or SDXC card into the SD card slot if your Mac has one.
  2. Launch Disk Utility, from the /Applications/Utilities folder, then click Continue.

Erase and reformat the storage device

  1. Click the attached storage device where it indicates its size in GB.
  2. Click the Partition tab.
  3. Select "1 Partition" from the Partition Layout pop-up menu.
  4. Click Options. Note: Do not select more than one partition.
    partition options
  5. Click the GUID Partition Table radio button, then click OK.
    partition options
  6. In the format field, select Mac OS Extended (Journaled).
    format menu
  7. Click Apply.
  8. Enter a name for the drive in the Name field.
  9. Verify that you wish to erase and reformat the selected drive by clicking Partition.
  10. After the reformatting process is finished, choose Quit Disk Utility from the Disk Utility menu.

Install OS X

Open the Mac App Store application from the Apple () menu. The Mac App Store normally displays OS X as being "Installed" in the purchases pane. This prevents you from downloading it when your computer already has the same version of OS X installed.

To re-download the installer, press and hold the Option key as you click the Purchases tab. The button to the right of the item you want to download should change to say "Install". If it does not, use Spotlight to search for "Install OS X" on your computer. It's likely you already have a copy of the same OS X installer on a connected drive. (It's normally located in the Applications folder.)

Once you have downloaded and located the installer, use these steps to install OS X on the external volume:

  1. Launch the OS X installer you downloaded from Mac App Store. The installer is normally located in the Applications folder.
  2. Follow the onscreen instructions to complete the OS X installation.
  3. When you are asked to select a drive for installation, click the button "Show All Drives". Be sure you select the external storage device you've connected for this purpose, not your computer's normal startup drive.

Once installation to your external device is complete, you can use it to start up your computer. You can then use Disk Utility, the Mac App Store and the OS X installer to install, or erase and install OS X on the normal startup drive of your computer. Even if you cannot create a Recovery System on your normal startup drive, you can reinstall or repair it at a later date if needed by connecting the external drive you just prepared. Simply hold down the Command and R keys (Command-R) while restarting your computer to start up from the external Recovery System.

Additional Information

  • The steps in this article erase and reformat the storage device.
  • Your storage device must have at least 13 GB available (after formatting) to install OS X Lion or later and a Recovery partition.
  • This article instructs you on setting up a storage device to use the GUID partition scheme and the Mac OS Extended (Journaled) format, which are required to install OS X and a Recovery partition on your external storage device.
  • You should back up any important files that are on the external storage device to a different drive.
  • This procedure installs a version of OS X that is compatible with the Mac it was created with. Using this OS X system with a different Mac model may produce unpredictable results.
  • Your computer's serial number is sent to Apple as part of this process to help authenticate your request to download and install OS X.
Last Modified: Oct 22, 2013
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  • Last Modified: Oct 22, 2013
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