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OS X has many options that can be used when the system is turned on. Start holding the keys down when the computer turns on, about the same time the startup noise occurs. Hold the keys until the progress indicator appears. The following commands can be used to help troubleshoot or upgrade an Apple computer.

• Press X during startup – Force Mac OS X startup
• Press Option-Command-Shift-Delete during startup – Bypass primary startup volume and seek a different startup volume (such as a CD or external disk)
• Press C during startup – Start up from a CD that has a system folder
• Press N during startup – Attempt to start up from a compatible network server (NetBoot)
• Press T during startup – Start up in FireWire Target Disk mode
• Press Shift during startup – Start up in Safe Boot mode and temporarily disable login items and non-essential kernel extension files (Mac OS X 10.2 and later)
• Press Command-V during startup – Start up in Verbose mode
• Press Command-S during startup – Start up in Single-User mode

OS X keyboard shortcuts for once the operating system has been loaded can be found here:
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