Resetting the EFI (NVRAM or PRAM) on a MAC
If you’re having trouble starting up your Mac, briefly seeing a question mark during boot, or finding that your Mac constantly boots from the wrong hard drive, resetting the EFI or clearing the NVRAM can help
The EFI (extensible firmware interface) is a set of instructions stored in the computers memory (NVRAM or PRAM) that act as an interface between the computer components and the operating system during startup.
Although it is mainly used in Intel-based Macintosh computers, other computer manufacturers are beginning to replace the older BIOS (Basic Input/Output System) firmware with EFI.
If you are having startup problems or your computer isn’t working properly, you may be able to solve the problem by resetting the EFI to its default settings.
1. Shut down the computer.
2. Turn on the computer and immediately hold down the “Command,” “Option,” “P” and “R” keys at the same time.
3. Release the keys after you hear the computer beep twice.
4. Check to see if the computer is working.
If the computer doesn’t start after doing all of the above then try this…
1. Reboot the computer and hold down the “Command,” “Option,” “O” and “F” keys at the same time during startup.
2. Wait until you see the message “To continue booting, type ‘mac-boot’ and press return.” If the message does not appear, try starting the computer by holding down the power button continuously until the message appears.
3. Type “reset-nvram” and press “Return.”
4. Type “reset-all” and press “Return.”
How to See What’s In Your NVRAM
You can find out what’s actually stored in your NVRAM by using the following procedure
Open up the Terminal in your macOS, which you’ll find in Applications > Utilities.
nvram -xp, then press Enter and you will be able to see the complete contents of your NVRAM.
While you have the Terminal open, you can clear the NVRAM using the command
Then restart your Mac for the reset to complete.
Clearing the NVRAM won’t solve all of your problems, but it can solve some, especially if you’re having trouble getting your Mac to boot.