Back in the days of the Classic MacOS, one of the most powerful keys to hold down at boot was the space bar. Introduced in System 7.5, Extensions Manager allowed users to select sets of extensions and troubleshoot conflicts in the middle of the boot process. Holding down Apple and Option at boot rebuilt the desktop. Thankfully, those days are long gone.
MacOS X doesn’t require any such maintenance, but like all operating systems, corruption does occur. Here are a few tips.
Holding down the option key at power-up brings you into the boot manager. If you have a bootable external drive connected, it will show up in the boot manager. This is helpful if you wish to select a boot device without booting the computer all the way, and switching the preferences in Startup Disk preference pane.
Similarly, Apple-Option-Shift-Delete bypasses the internal hard drive and tells the computer to use an external device. A nice time saver when you know what you want to boot from!
Holding down Apple-S on your keyboard at boot will bring you into Single User Mode. You can do plenty of harm here, but there’s one command that’s very safe: fsck -yf . This is the same as using the Repair Disk function of Disk Utility, and is an indispensable tool when your machine won’t boot. If this process is unable to repair your disk, you’ll need something stronger like DiskWarrior. And if that doesn’t help, the best software-based recovery utility in my mind is Data Rescue II. We use both extensively in our service department.
Just like the Classic MacOS, holding down Shift at boot will put your computer into Safe Mode. In the process, the same fsck (file system consistency check) will be run while at the gray apple screen. The computer will proceed to boot without third party software and nonessentials like AirPort drivers. If your machine won’t boot in Safe Mode, it’s time to consider DiskWarrior. It might even be time for professional diagnosis.
Another tip is to not hold down the power button too long. A quick press is all it takes. If you hold it down too long, the computer will look for a firmware update, and may not boot unless restarted.