If you're an advanced user who’s comfortable with UNIX, you can use single-user mode or verbose mode to help isolate issues related to startup.
Verbose mode (pictured) provides additional details as to what the computer is doing and what drivers and software it is loading during startup. This level of detail can be very helpful for troubleshooting problems with hardware or software, if errors are occurring during startup or after the operating system has loaded.
To start your Mac in single-user mode or verbose mode:
- Hold down one of these key combinations on your keyboard immediately after pressing the power button to turn on your Mac, or immediately after your Mac begins to restart: Command-S for single-user mode or Command-V for verbose mode.
- Continue holding the keys until you see white text on the screen.
- If you're using FileVault, release the keys when you see the login window. Then log in to continue startup in single-user or verbose mode.
- If you're using a firmware password, you must turn off the password before you can start up in single-user or verbose mode.
You can now enter UNIX commands. To exit this mode and start up normally, type reboot, then press Return.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)