Safe Mode in OS X is a limited boot environment where only essential system services are active. It is a great option to have when troubleshooting software problems on your system, and when you invoke it by holding the Shift key at startup, your Mac should show a gray progress bar and then an indication in the menu bar that you are in Safe mode. This mode should only be active when you specify it by holding the Shift key at bootup; however, there may be problems that can cause your Mac to always boot into Safe Mode.

Unplug USB devices

The invocation of Safe Mode is usually done through a boot argument variable that is passed to the system kernel from your keyboard (ie, you hold Shift at startup). Therefore, first try troubleshooting the situation by unplugging your keyboard and all other USB devices when starting up your Mac. If the system boots normally, then focus on your keyboards and other USB devices to see which one may be triggering the Safe Boot.

Reset PRAM boot arguments

While your keyboard is the primary way to invoke Safe Mode, you can also do so using the “boot-args” PRAM variable that can be set in the Terminal with the following command:

sudo nvram boot-args="-s"

Unlike the keyboard which requires you hold the Shift key specifically to boot to Safe Mode, once this PRAM variable is set, then the system will always read it and boot to Safe Mode. Therefore, try clearing the PRAM to ensure this or other odd variables that might have been set there are cleared. To do this, you can reboot your system and hold the Option-Command-P-R keys immediately after the boot chimes, followed by releasing them once the system resets and sounds the boot chimes again, or you can open the Terminal and specifically clear the “boot-args” variable by running the following command:

sudo nvram boot-args=""

Verify and Repair your hard drive

Often a system that is stuck in Safe Mode is being sent there by the system because the system detects a problem on the filesystem and is trying to repair it. Therefore, try booting into Recovery Mode and see if you can use Disk Utility there to repair your hard drive. Even if your drive is showing OK health in Disk Utility, consider using a robust third-party alternative like Disk Warrior, to ensure your drive’s formatting is correct.

Format and restore from backup

Optional to purchasing a third-party utility (which may be close to $100 or more), is to clear any potential formatting problems by simply reformatting your hard drive. First ensure you have a full backup of your system (more than one is preferable), and then perform these steps:

1. Boot to Recovery Mode by holding Command-R at startup.
2. Choose “Disk Utility” from the tools window.
3. Select your hard drive device (above the boot volume name) in the Disk Utility sidebar.
4. In the Partition tab choose “1 Partition” from the Partition Layout menu.
5. Click the “Options…” button and choose “GUID” as the partition type, and then click “Apply”
6. Quit Disk Utility

After these steps have been done, you can restore your Mac from your backup. To do this with Apple’s Time Machine backup service, you can click the option to restore from backup in the main Recovery tools window. Then follow the on-screen instructions.

Reinstall OS X

As a final step after trying the above options, consider a simple reinstall of OS X, which will replace your system software with a fresh version, while leaving your data, applications, accounts and settings, and other information intact. This will ensure no problem with your OS installation is spurring the constant Safe boot.

To do this, again boot your Mac into Recovery mode, but this time do not format or restore from backup, and instead just choose the first option to reinstall OS X. Follow the on-screen instructions, and wait for the installation to complete. Once installed, be sure to first go to Software Update in the Apple menu and apply any updates you see there, as this will update your system to the latest OS version.