90. Single User Mode
Single User Mode bypasses a number of standard elements of Mac OS X and puts the user straight into the console where they can make changes using command line utilities. This mode is primarily used to fix problems that occur during startup or when a Mac fails to boot properly.
In order to launch Single User Mode, the Mac needs to be restarted and the Command and S keys held during startup as soon as the startup chime is heard. A wide range of useful Single User Mode commands can be found on Apple's support pages.
91. Hardware Test
For access to advanced troubleshooting tools if your Mac is exhibiting errors or not booting correctly, you can load the Apple Hardware Test from the install DVD that came with your Mac. With the disc in the drive, boot up your Mac and hold down the D key when you hear the startup chime.
The Apple Hardware Test enables you to scan your discs and drives as well as other components to detect any problems with your computer's hardware. This information can then be provided to tech support for advice on replacements and fixes.
92. Reset PRAM & NVRAM
The acronym-suited mouthfuls Parameter Random Access Memory and Non-volatile Random Access Memory refer to special stores of information for Mac OS X such as time-zone settings, startup volume, speaker volume, DVD region settings and more.
Resetting the NVRAM or PRAM can fix a number of Mac problems including startup issues. In order to reset the values, hold down Command+Option+P+R as soon as you hear the startup chime. If done correctly, your Mac will restart and you'll hear the chime again. Now the PRAM or NVRAM will have been reset and you can release the keys.
93. Safe Boot Mode
Safe Boot Mode is often the only way to boot into Mac OS X if the boot drive is encountering problems. This is normally exhibited by the startup hanging at the default grey screen, preventing OS X from booting.
By launching into Safe Boot Mode via holding down the Shift key during startup, your Mac forces a directory check of the startup volume and only launches the bare minimum of kernel extensions, fonts and startup items required, so as to avoid any that may be causing problems and allow for easier identification.
Of course, safe Boot Mode also enables you to make changes to prevent problems when booting your Mac in the future.
94. Eject CD
If you have a disc in your drive that is refusing to eject, restart your computer and hold down the mouse button as soon as you hear the startup chime. This should eject your disc while your Mac starts up.
95. Startup Manager
If you're having trouble with your hard drive and need to boot your Mac from a different drive or disc, hold down the Option key during startup to open the Startup Manager, which will enable you to select a different volume.
96. Start From Disc
To boot up your Mac from an install disc or other bootable CD or DVD, insert the disc and then hold down the C key while the computer starts up. Your Mac will then look for and boot from the disc.
97. Target Disc Mode
Holding T during startup will tell your Mac to mount its drive on the desktop of a host computer connected via FireWire cable. This can be used to copy files between Macs or for troubleshooting a disc fault.
98. Verbose Startup
Sometimes you may need to start your Mac in Verbose mode to show all system messages normally hidden during startup. Hold down Command+V at startup.