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OS X can start up your Mac in a myriad of ways, some of which our ongoing tutorial series has discussed already in detail such as using built-in Startup Manager to pick a disk to startup your Mac from and booting into Safe, Verbose and Target Disk modes.

Today we get to talk about starting your Mac up from external storage like optical media or an external USB-based hard drive or flash storage.

Booting from an external storage allows you to start up your Mac in another version or copy of OS X, troubleshoot certain issues and more.

Requirements

Starting up your Mac from an external disk requires the following:

  • Intel-based Mac
  • Bootable optical media (CD or DVD) on Macs equipped with Apple’s Super Drive or a USB thumb drive formatted with a GUID partition type and containing an OS X installer or a usable operating system.

Apple advises against booting from external storage containing a version of Mac OS X earlier than the one your Mac shipped with.

How to start up your Mac from a bootable CD/DVD

Step 1: Turn on your Mac by pressing the power button, or restart it if it’s already on by choosing Restart in the Apple menu.

Step 2: Press and hold the C key on your keyboard immediately upon hearing the startup chime. Your Mac should start up from the OS X installer CD/DVD media. You can also press the left mouse button on a wired mouse during the startup in order to open the disc tray.

Tip: You can easily burn a bootable image of OS X to a DVD using Disk Utility.

How to start up your Mac from an external USB storage device

If your USB thumb drive, USB-based external disk or flash storage contains a usable operating system or an OS X installer, you can select it at boot time using OS X’S built-in feature called Startup Manager, which can be invoked via a simple keystroke.

Step 1: Turn on your Mac by pressing the power button by choosing Restart in the Apple menu, or restart it if it’s already on.

Step 2: Press and hold the Option (⌥) key immediately upon hearing the startup chime. Release the key after Startup Manager appears. Startup Manager will scan and list connected drives and volumes that can be booted from, as shown below.

OS X Startup Manager Mac screenshot 001

Step 3: Highlight the volume you want to use by using your mouse or trackpad, or left and right arrow keys on the keyboard.

Step 4: Double-click or press the Return key to boot your Mac from the selected volume.

Tip: If there are several USB storage devices connected to your Mac, Startup Manager will only list the ones containing bootable volumes.

Full tutorial: how to pick a startup disk for your Mac at boot time.

How to start up your Mac from an external volume using System Preferences

You can use System Preferences to tell your Mac to automatically use optical media or an USB-based external disk or flash storage every time it’s restarted.

Step 1: Open the System Preferences application in the Dock.

Step 2: Now click the Startup Disk pane. You will be presented with various disk icons.

OS X Startup Disk

Step 3: Select the system you would like to use to start your computer up.

Step 4: Now click the Restart button to start up your Mac using the chosen volume.

Tip: Your Mac will now always boot into the selected startup disk until you change it in System Preferences, or temporarily override it through Startup Manager.

Full tutorial: how to choose a startup disk using System Preferences.

Troubleshooting startup issues

Should you run into problems whilst attempting to boot your Mac from an external storage device, consult our FAQ below to help troubleshoot any start up issues you may be plagued with.

Is the storage device to an external power supply?

Depending on its model and power requirements, it may be necessary to connect your USB storage device to an external power supply.

Is the storage device connected and turned on?

Make sure your external storage is turned on and connected to your Mac. USB-based external hard disks usually have the power on/off button at the back of the enclosure.

Does your disk need repair?

If your Mac doesn’t boot from an external volume or the drive doesn’t show up in Startup Manager even though it contains a bootable copy of OS X, it might need repair.

The first order of business should be attempting to repair disk permissions in Disk Utility. In the worst case scenario, you might need to reinstall OS X on the external volume.

Did you try using a different USB port?

If you own a MacBook or MacBook Pro notebook, try using a different USB port if you cannot startup from the external USB storage device.

Has the disk been formatted with a GUID partition type?

Intel-based Macs support starting from an external USB storage device’s volume that has been formatted with a GUID partition type.

Does your Mac shut down automatically upon connecting the disk?

Apple states that a MacBook Air contains a preventive measure which prompts the computer to shut down automatically if the connected USB storage device has issue or if the USB port on the MacBook Air itself is damaged.

Are you using Boot Camp?

If you’re using Boot Camp in a dual-boot Windows/OS X environment, you may be unable to boot natively into supported versions of Microsoft Windows XP, Windows Vista or Windows 10 operating systems installed on external USB hard drive.

Are you using an external USB hub?

Using an external hub to connect several USB storage devices may cause your Mac to fail to recognize them at boot time. Try connecting the USB drive to another port on the hub. Should that fail, connect the disk directly into your Mac’s USB port. Keep in mind that some USB hubs and external USB storage devices need an external power supply, depending on their power requirements.

Is the disk connect to the keyboard’s USB port?

Both wired and wireless Apple Keyboard, and some other keyboards with built-in USB ports, may not supply enough power to some external USB hard drives.

Is the disk bootable?

Make sure the external drive you’re trying to start from contains a usable operating system. Volumes that aren’t bootable and don’t contain a copy of a valid operating system aren’t listed in Startup Disk or Startup Manager.

Are you using wireless keyboard?

If you use an Apple Wireless Keyboard or another Bluetooth keyboard with your Mac, keystrokes won’t be recognized during the startup process because Bluetooth isn’t initialized at boot time. Connect a wired keyboard or use the built-in keyboard if you own a Mac notebook. The same disclaimer applies to holding the left button on a wireless mouse at boot time to open the disc tray.

Other ways to start up your Mac

Your Mac can start up in a number of different ways, including the following methods that our tutorials provide step-by-step instructions for:

  • Safe ModeInvoked by pressing and holding the Shift (⇧) key after hearing the startup chime, Safe Mode disables certain features of OS X and loads only essential components to make troubleshooting software issues simpler. It also includes an automatic disk check and repair which might resolve boot problems and allow you to start up your computer successfully.
  • Verbose ModeVerbose Mode is a text-only environment which displays status messages as the startup process progresses. Press and hold the Command (⌘) – V key combination after hearing the startup chime to enter Verbose Mode. This mode is commonly used to troubleshoot startup issues and resolve problems with kernel extensions and custom hardware while allowing you to see exactly what’s going on behind the scenes as OS X loads its components.
  • Target Disk ModeTarget Disk Mode allows a Mac to be used as an external hard disk connected to another Mac. To enter Target Disk Mode, turn on your Mac and hold down the T key immediately upon hearing the startup chime. Once the computer is in Target Disk Mode and available as an external volume to the host Mac, you can copy files to or from that volume. Any Mac with a FireWire or Thunderbolt port can be started up in Target Disk Mode.
  • Startup ManagerOS X’s built-in Startup Manager allows you pick a volume to start your Mac up from at boot time, which temporarily overrides your startup disk defaults set in System Preferences. Just turn on your Mac, hold the Option (⌥) key after the startup chime and release it after Startup Manager appears.
  • System PreferencesTo change your default startup disk, use the Startup Disk pane in the System Preferences application. Your Mac will boot into the selected disk until you choose another one or temporarily override your selection using Startup Manager at boot time.

We will be covering more ways to start up your Mac in future tutorials.

See also:

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  • mreo

    This article failed to mention that “Pressing the ‘C’ Key on your keyboard is useless if the keyboard is not a wired one.

    If your keyboard is of the wireless Bluetooth variety; pressing the ‘C’ key will not help you in any way – as the bluetooth signal is not an initial boot-up item.

    Same goes with pressing the mouse button during start-up to open the disc tray; if your mouse is of the wireless variety, the command will not work at all.

    In both cases, the peripherals MUST BE of the WIRED variety.

    Details people, details make all of the difference!

    • That’s true.

      Thanks for the heads-up: I have added that little detail to the article.

  • Andrew

    “Turn on your Mac by pressing the power button by choosing Restart in the Apple menu, or restart it if it’s already on.”

    Wait, what?

    “You can also press the left mouse button on a wired keyboard…”

    Wait, what what?

  • Calvin

    I have a wireless keyboard and the key commands all work for me.