Does your Mac get stuck during the startup process? If so, then you might want to try Safe Boot to see if you can get your Mac to startup successfully. Safe Boot is the process of making your Mac start up with a bare minimum of system software. It also forces an inspection of the hard drive.

Once the Mac has finished safe booting it is in Safe Mode. Safe Mode can be useful for troubleshooting a number of problems. Here are instructions to initiate a Safe Boot:

1. Be sure your Mac is off. If it’s on but unresponsive, you can force it to turn off by holding its power button for 5 seconds.
2. Unplug everything you can from the back of the Mac, especially USB devices. Disconnect any external hard drives and even the ethernet cable, if one is connected.
3. Press and release the power button. This turns on the Mac.
4. Immediately after you hear the startup chime, press and hold the Shift key.
5. Continue to hold the Shift key until you see the gray Apple logo and a progress indicator. On older versions of macOS  the indicator looks like spinning gear. On newer versions of macOS the indicator is a grey progress bar.
6. After releasing the Shift key, I recommend walking away to give your Mac extra time to startup. A Safe Boot takes longer than a regular boot. Often it only takes a few extra minutes, but one client reported that it added 30 minutes!

If your Mac completes a Safe Boot then restart it and see if boots normally now. If so, then most likely you had a corrupted cache or some damaged directories that were fixed by the Safe Boot. Reconnect your external devices and restart. If the Mac fails to start up then one of these devices is causing a problem.

If your Mac gets stuck during a Safe Boot then contact an Apple Consultant or the Apple Store. There’s a chance that your Mac has a hardware problem. For example, its internal hard drive could be failing. (Your Mac is backed up, right?) Another possibility  is that your copy of OS X is badly damaged. You could start your Mac in Verbose Mode to try to learn, more specifically, at which stage your Mac is getting stuck.

Don’t try to use your Mac in Safe Mode. Some features are disabled in this mode, such as file sharing, the DVD player, Quartz Extreme (hardware accelerated graphics) and some wireless network cards. Apple’s article on Safe Boot provides more detail about disabled features.