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From the book Reference 29.4

Reference 29.4
Startup Shortcuts

Your Mac firmware supports many keyboard shortcuts, which, when pressed and held during initial power-on, allow you to modify the startup process. Some of these shortcuts modify the booter selection, while others modify how OS X starts up. These alternate startup and diagnostic modes are often used for troubleshooting system issues.

Select Alternate Systems

Mac startup shortcuts that allow you to select another system include:

  • Option—Starts up into the Startup Manager, which allows you to select any volume containing a valid system to start up from. This includes internal volumes, optical disk volumes, some external volumes, and, on later Mac models, Wi-Fi networks and NetBoot images.
  • C—Starts up from a bootable CD or DVD in a supported optical disk.
  • D—Starts up from the Apple Hardware Test partition on the first restore DVD included with your Intel-based Mac. Later models also include this diagnostic built into the hardware ROM and thus don’t require the DVD.
  • Command-Option-D—Starts up from the Apple Hardware Test via an Internet connection to the Apple servers. This option is only available to Mac models released after July 19, 2011.
  • N—Starts up from the last-used NetBoot server, or the default NetBoot server if none was previously used. The Mac shows a flashing or spinning globe icon in the center of the main display until it locates the NetBoot server, at which point it shows the dark gray Apple logo.
  • Option-N—Similar to holding the N key, but this shortcut always starts up from the current default NetBoot server instead of the last-used NetBoot server.
  • Command-R—Starts up from the local OS X Recovery, if available. If no local OS X Recovery was found, Mac systems that support OS X Internet Recovery automatically attempt to start up from the Apple recovery servers. Lesson 4, “OS X Recovery,” covers this topic in greater detail.
  • Command-Option-R—Forces startup to OS X Internet Recovery on supported Mac systems.

Modify OS X Startup

Mac startup shortcuts that modify the OS X default startup:

  • Shift—Starts up OS X using Safe Boot, which leaves OS X running in safe mode. During Safe Boot, the system clears specific caches, carefully tests startup procedures, and limits automatically launched processes during each stage. While running in safe mode, many non-essential system and third-party items are ignored. Details regarding Safe Boot and safe mode are covered throughout the troubleshooting sections later in this lesson. You can recognize a system that has performed a Safe Boot and is now in safe mode if the words “Safe Boot” appear as bright-red text in the top right corner of the login screen. You can also verify after login by opening the System Information application and then selecting the Software item.
  • Command-V—Starts up OS X in verbose mode. In verbose mode, the system does not hide the startup progress from you with the light gray screen. Instead, you see a black background with white text showing all details of the startup process.
  • Command-S—Starts up OS X in single-user mode. When starting up in single-user mode, the system only starts core kernel and BSD UNIX functionality. You must be familiar with the command-line interface to use single-user mode.

Other Startup Utilities

Other useful Mac startup shortcuts:

  • T—For Mac systems with built-in Thunderbolt or FireWire ports, holding down this key powers on the Mac in target disk mode, allowing other computers to access your Mac computer’s internal drives. Target disk mode details are covered in Lesson 13, “File System Troubleshooting.”
  • Command-Option-P-R—Resets NVRAM settings and restarts the Mac.
  • Eject key, F12 key, mouse, or trackpad button—Ejects any removable media, including optical discs.

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