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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.
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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.
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Step 3: Highlight the volume you want to use by using your mouse or trackpad, or left and right arrow keys on the keyboard.
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Step 4: Double-click or press the Return key to boot your Mac from the selected volume.
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Tip: If there are several USB storage devices connected to your Mac, Startup Manager will only list the ones containing bootable volumes.
How to boot up your Mac from your new bootable USB flash drive.
Start up options for your Mac
Your Mac can start up in a number of different ways, including the following methods:
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Safe Mode: Invoked 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.
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Verbose Mode: Verbose 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.
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Target Disk Mode: Target 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.
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Startup Manager: OS 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.
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System Preferences: To 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.