On a Mac, how do I change my startup volume or boot from an alternative volume, disc, or device?

Depending on what you're specifically trying to do, you have the following options:

• Permanently changing your startup volume: While running in Mac OS X, select System Preferences...  from the Apple menu, and then choose Startup Disk. Choose your startup volume and then restart. Unlike with the other options in this list, this new volume will be the default until you change it.
  
  
• Accessing the startup manager: Restart your computer and immediately press the Option key. Icons for all available startup volumes will appear. Use your arrow keys to select the one you want to boot from, and then press Return.
  
  
• Booting from the recovery volume: In Mac OS X 10.7 and later, restart your computer and immediately press Command-r. The computer will boot from your Recovery HD volume, which will allow you to perform diagnostics or reinstall the operating system.
  
  
• Booting from a startup CD or DVD: Restart your computer and immediately press the  c  key. The computer will look for a bootable CD or DVD and, if it finds one, will use it as its startup device.

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