Macs store important information and settings in NVRAM. These settings include information that affects the speaker volume, screen resolution, startup disk selection and any kernel panic information that can aid in troubleshooting. When your Mac won't boot, you can attempt several troubleshooting steps to correct the problem. Only Intel-based Macs use NVRAM; older PowerPC-based models use a similar technology referred to as PRAM.
Before resetting NVRAM you must shut down your Mac. Once you shut down your Mac, press the power button and then immediately hold down "Command-Option-P-R." You can use the "Command" and "Option" keys on either side of the keyboard. When you hear your computer start up for the second time you can release the keys. If you have a MacBook, the light on the MagSafe charger may blink. This process resets your NVRAM, but you might need to reconfigure your volume settings, screen resolution, time zone and startup disk options.
Startup Disk Selection
The NVRAM provides the information that tells the operating system the startup disk to use. You may also have to select a new startup disk after resetting your NVRAM. If your computer does eventually boot, you can select a new startup disk using System Preferences. Select the "Startup Disk" option, click the startup disk you want to use and press the "Restart..." button. Otherwise, you need to hold down the "Option" key while your Mac starts. After a few moments, the Startup Manager displays on the screen. Use the left or right arrow keys to move between the available hard disks and press the "Return" key to select your startup disk.
OS X Recovery
If your Mac still doesn't start normally, disconnect any external hard disks and peripherals such as printers and scanners. Shut down your computer and hold down "Command-R" while the computer boots until OS X Recovery launches. Select "Disk Utility" and click the "Continue" button. Choose the main hard disk from the list and click the "Repair Disk" option. If this still doesn't resolve the issue, launch OS X Recovery again and select the option to reinstall Mac OS X Mavericks. Reinstalling does not delete any of your files, applications or data, but it can help restore missing files and repair other issues that may prevent your Mac from starting.
Logic Board Battery
NVRAM relies upon a small battery in the logic board to supply its power. When the battery begins to die, you may start to notice that your settings get lost or your Mac takes longer to start. When you can't resolve the issue by resetting the NVRAM and taking other troubleshooting steps, you should take your computer in for repair. A specialist can replace the battery and diagnose other issues that might cause your computer to hang at startup.
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