For the most part, starting up your Mac involves nothing more than pressing the power button, and away you go. But there are a few options for modifying how your Mac starts up – if you know which keys to press. 
The Mac OS X Finder
Here’s how this works: Start up your Mac, and as soon as you hear the familiar chime, press and hold the modifier key combination to alter what happens when Mac OS X boots.
- C The C key tells your Mac to bypass its default startup volume and boot from a CD or DVD with a system folder.
- T Use the T key to boot into target disk mode. This lets you plug your Mac into another Mac with a FireWire cable. Your Mac will appear as a hard drive on the other Mac’s Desktop.
- Shift Holding down the Shift key after startup disables non-essential kernel extensions and login items until your reboot. This is called Safe Boot, and is handy for troubleshooting system-related problems. If the issue goes away during a Safe Boot, odds are it is related to a login item, or a bad kernel extension that an application installer tossed into your system folder.
Variation on a theme: Wait until after you see the blue startup screen, and then hold down the Shift key. This lets all system-level extensions and login items load, but prevents anything that loads when you log in from launching.- Command-Option-Shift-Delete This finger-origami keyboard combination tells your Mac to bypass your default startup volume and look for any bootable disk, CD, or DVD it can find.
- Command-S The Command-S keyboard combination tells your Mac to boot into Single User Mode, which is actually a Unix command line. This is another useful key combo for troubleshooting system problems. If you aren’t comfortable mucking around a Unix command line, this is a good one to avoid.
- Command-V Verbose Mode, invoked with the Command-V keyboard combination, was something I relied on all the time in the early and rough days of Mac OS X since it lists everything that is loading and launching during the startup process. It was a lifesaver for troubleshooting a horked Mac OS X 10.1 installation on many occasions for me.
- X Holding down the X key during startup was also handy during the early days of Mac OS X when we had Macs that could also boot Mac OS 9. If your Mac OS 9 System Folder was damaged and your Mac wouldn’t boot, you could use the X key to force your Mac to boot Mac OS X.
If there is another startup keyboard sequence you like to use, feel free to share it below.
- T Use the T key to boot into target disk mode. This lets you plug your Mac into another Mac with a FireWire cable. Your Mac will appear as a hard drive on the other Mac’s Desktop.