Sometimes your Mac’s parameter RAM (PRAM) becomes scrambled and needs to be reset. This can cause startup problems for your Mac. This small piece of memory isn’t erased or forgotten when you shut down your Mac.
A small amount of your computer’s memory, called “parameter random-access memory” or PRAM, stores certain settings in a location that OS X can access quickly. The particular settings that are stored depend on your type of Mac and the types of devices connected to it. The settings include your designated startup disk, display resolution, speaker volume, and other information.
- Shut down your Mac.
- Locate the following keys on the keyboard: Option, Command (⌘), P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on your Mac.
- Immediately press and hold the Option-Command-P-R keys. You must press this key combination before the gray screen appears.Continue holding the keys down until your Mac restarts, and you hear the startup sound for the second time.
- Release the keys.
If you have a Redundant Array of Independent Disks (RAID) scheme set up, your computer may not start up if you reset parameter random-access memory (PRAM) when you restart.
If this happens, hold down the Option key while you restart your computer. Then select your startup disk when you see the prompt. After you successfully start up your computer, choose Apple menu > System Preferences, click Startup Disk, then select your startup disk.
Resetting PRAM may change some system settings and preferences. Remember that your chosen startup disk, time zone, and sound volume are reset to their default values when you zap your PRAM. Use System Preferences to restore your settings.