to Reset the EFI on a MAC
The EFI (extensible firmware interface) is a set of instructions
stored in the computers memory that act as an interface between
the computer components and the operating system during startup.
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.
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.
Shut down the computer.
Turn on the computer and immediately hold down the "Command,"
"Option," "P" and "R" keys at the
Release the keys after you hear the computer beep twice.
Check to see if the computer is working.
the computer doesn't start after doing all of the above then try
the computer and hold down the "Command," "Option,"
"O" and "F" keys at the same time during startup.
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.
Type "reset-nvram" and press "Return."
Type "reset-all" and press "Return."