When a problem arises in OS X because of a software configuration error, often one troubleshooting step to take for clearing the issue or at least investigating it further is to boot into Safe Mode; however, sometimes Safe Mode may not work.
Booting to Safe Mode in OS X is done by holding the Shift key at startup, which triggers a minimal boot environment where only basic Apple-supplied extensions and services are loaded. This will help you determine if problems are happening from peripheral devices or third-party extensions such as those supplied by some security software packages. In addition to a minimal environment, Safe Mode runs a directory check on the boot drive and clears some temporary caches, which, if corrupted, can cause slowdowns and other errors in the system.
While in a Mac's default configuration, Safe Mode will work, there are some instances where special configurations or errors may prevent it from working.
- Firmware passwords
As an extra security measure in Mac systems, Apple provides the option to lock the system with a firmware password. This can be done using the Firmware Password utility that's available in an OS X recovery boot drive, be it an older OS X boot DVD or a modern Recovery HD partition.
If a firmware password is set on your system, then you will not be able to boot to Safe Mode or other modes like single-user mode or verbose mode. You will also not be able to perform any hardware resets such as resetting the PRAM, or choose an alternative boot volume without first supplying the firmware password.
Therefore, to test if your system has a firmware password set, try booting with the Option key held down to get to the boot menu. If you see the menu appear and display your main boot drive, then you do not have a firmware password, but if you see a password prompt appear, then you do have one set and will need to use the firmware password utility to remove it before Safe Mode will work.
Do keep in mind that while a forgotten firmware password could easily be reset in older Mac systems by removing hardware components like RAM, Apple has changed this in newer systems. For models from around mid-2010 through early 2011, if you set a firmware password and forget the password, then you will have to take the system in to an Apple store to have the password reset.
- Clear PRAM
As with other special boot modes, Safe Mode can be influenced by the system's parameter RAM (PRAM) which is a small memory allocation that stores hardware-based variables for use by the system before the operating system loads. Part of the PRAM's contents is a boot argument variable that can contain flags for booting to Safe Mode among other modes, and resetting the PRAM will ensure these flags are clear from influencing how the system boots.
To reset the PRAM, reboot your system and immediately hold down the Option, Command, P, and R keys all at once. Hold them until the system automatically reboots (when it does this the volume of the boot chimes may be louder or softer), and then release them and allow the system to boot normally.
- Try a different keyboard
Sometimes errors in the keyboard or in the USB connection for it may prevent the system from accepting boot commands sent to it. Therefore, you can try troubleshooting the keyboard to see if you can get the Shift key press to be accepted at startup. One option here is to try a different USB connection on your Mac, and another option is to try another keyboard altogether. Any USB keyboard should work just fine, so you can try one from another Mac, or one from a Windows PC.
- Disable FileVault in Lion
In OS X Lion Apple implemented a new full-disk encryption technology called FileVault (or more appropriately "FileVault 2"), which first preboots the system to a log-in screen that requires you supply your password to unlock the drive before the system can boot. This configuration prevents the system from accepting a keyboard-based boot command like the Shift key and passing it to the operating system.
Apple has acknowledged this limitation with FileVault, and affirms that the only option for booting to Safe Mode if you have FileVault enabled in Lion is to first disable it in the Security system preferences. This may take a little while to do, but should not affect your data at all.
Do keep in mind that while this is the case for people using FileVault 2 in Lion, it is not the case for the first-generation FileVault technology in earlier versions of OS X.