Under Mac OS X, you can use several startup options. The most straightforward way is to use the Startup Disk pane of the System Preferences utility to choose a startup volume. There are other ways you can control system startup as well, such as choosing a startup volume during the startup process, starting up in the single-user mode, and starting up in the verbose mode.
The Startup Disk pane of the System Preferences utility enables you to choose a startup volume. Open the pane and you will see a list of the valid startup volumes on your machine. Select the volume from which you want to start up and click Restart. You will be prompted to confirm this action by clicking the Save and Restart button. Your selection will be saved and your Mac will start up from that volume.
Under Mac OS X, you can choose the startup volume by holding down the Option key while the machine is starting up. When you do, you will see a window that displays each of the valid startup volumes on your machine. The currently selected startup volume will be highlighted. You can choose a startup volume by clicking it and pressing Return (you can also click the right-facing arrow icon to choose the startup volume).
You can refresh the list of valid startup volumes by clicking the circular arrow button.
The single-user mode starts up your Mac in a Unix-like environment. In this environment, you can run Unix commands outside of Mac OS X. This can be useful in a couple of situations, mostly related to troubleshooting problems.
To start up in single-user mode, hold +S while the machine is starting up. You will see many system messages that report on how the startup process is proceeding. When the startup is complete, you will see a date and time message, and near that you will see the localhost Unix prompt.
During the startup process, you are likely to see some information that doesn't make a lot of sense to you unless you are fluent in Unix and the arcane system messages you see. You will also probably see some odd error messages, but I wouldn't worry about them too much. However, if you have particular problems you are trying to solve, some of these messages might provide valuable clues for you.
One of the more useful things you can do is to run the Unix disk-repair function, which is fsck. At the prompt, type
and press Return. The utility will check the disk on which Mac OS X is installed. Any problems that it finds will be reported and repaired (if possible).
You can use many other Unix commands at this prompt, just as if you were using the Terminal application from inside Mac OS X.
To learn more about using Unix commands, see Chapter 9, "Unix: Working with the Command Line," p. 213.
To resume the startup process in Mac OS X, type the command exit and press Return. You will see yet even more arcane Unix messages and then the normal Mac OS X startup process will continue. When that process is complete, you will end up at the Login window or directly in the Mac OS X desktop, depending on how your Login preferences are configured.
If you hold down +V while your Mac is starting up, you will start up in the verbose mode. In this mode, you will see all sorts of system messages while the machine starts up. The difference between verbose mode and single-user mode is that the verbose mode is not interactive. All you can do is view the system messages; you can't control what happens. Many of the messages you will see will probably be incomprehensible, but some are not (particularly messages about specific system processes starting up). This mode is likely to be useful to you only in troubleshooting. And even then, the single-user mode will probably be more useful because it gives you some control over what is happening.