How to install OS X on an external drive connected to your Mac

You can install OS X on an external drive to use the device as a startup disk.

Using an external drive as your startup disk can be useful if you're an IT professional or need to test an issue. When your external drive is connected to your Mac, you can select it as a startup disk from the Startup Disk pane of System Preferences.

If your external drive is slower than the built-in startup disk, your Mac might take longer to start up or might perform slowly. If you're unable to start up your Mac from its built-in startup disk, you can reinstall OS X on it using OS X Recovery or the Mac App Store.

Before you begin

Your external drive needs a GUID partition with at least 8 GB of available space to install OS X. For best results, the volume should be formatted as Mac OS X Extended format (not FAT, NTFS, or EXFAT).

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. Then use these steps to make sure you can install OS X onto your device:

  1. From the Apple menu, choose About This Mac.
  2. Click the storage tab in the About This Mac window.
  3. Note the amount of free space on your external drive. It needs to be at least 8 GB.
  4. Click the Overview tab, then click the System Report button.
  5. Select "Storage" in the System Information window.
  6. Locate your external drive in the list of connected storage devices. Make sure the Physical Volumes list shows a Partition Map Type of "GPT (GUID Partition Table)."

If the external drive isn't partitioned as GUID, you can reformat it so that you can install OS X.

Install from OS X Recovery

To install OS X on an external drive using OS X Recovery, follow these steps.

  1. Start up your Mac. Immediately after you hear the startup sound, press and hold the Command+R on your keyboard.
  2. After you see an Apple logo or globe appear, you can release the keys. If you're prompted to select a Wi-Fi network, select a network that has a working Internet connection.
  3. When the Recovery screen appears, make sure you're connected to the Internet. If not, select a Wi-Fi network with a working Internet connection from the Wi-Fi menu.
  4. Select the option to Install OS X from the Recovery window.
  5. Follow the onscreen instructions to complete the installation. When you're 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 built-in startup drive.

Your Mac might restart one or more times during installation. After installation is finished, your Mac automatically starts up from the external drive.

When you're done using your external disk, select a startup disk from System Preferences to have your Mac to start up from its built-in startup disk or another drive.

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.

Install from your normal startup disk

If you can start up from your built-in startup disk or another drive, you can install OS X using the OS X Installer. First download a version of OS X from the Mac App Store that you own, then use the Install OS X app to install it on your external drive.

Download the OS X Installer from the App Store

Use these steps to download the version of OS X you want to install.

  1. From the Apple menu, choose App Store. 
  2. Hold the Option key and click the Purchases tab. You should see the versions of OS X you've previously purchased. If you don't see your purchases, log in to the App Store with the ID you previously used to install OS X. 
  3. Click the Install button next to the version of OS X you want to download.

The Mac App Store normally displays OS X as being "Installed" in the purchases pane. This prevents you from downloading it when your Mac already has the same version of OS X installed. When you 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 doesn't, 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 in the Applications folder, or on a connected drive. 

Install OS X

After you download the installer, use these steps to install OS X on your external drive:

  1. Use the Install OS X app you downloaded from Mac App Store. This installer automatically opens after you download it. If you closed the installer app and need to re-open it, you can find it in the Applications folder.
  2. Follow the onscreen instructions  in the Installer window to complete the installation. When you're asked to select a drive for installation, click the button "Show All Disks." Select your external drive, then click Continue.

Your Mac might restart one or more times during installation. After installation is finished, your Mac automatically starts up from the external drive.

When you're done using your external disk, select a startup disk from System Preferences to have your Mac to start up from its built-in startup disk or another drive.

Learn more

These steps install a version of OS X that's compatible with the Mac it was installed from. Using this external drive with a different Mac model might not work.

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