How to Force iMac to Boot EFI
The iMac has several special boot modes that allow you to boot the computer differently than has been previously set up in the Startup Disk preference pane. By default, all Intel Macs boot using Extensible Firmware Interface (EFI), but then immediately begin starting up off of the chosen Startup Disk. Holding modifier keys when launching your iMac can force these alternative options.
Shut down your iMac from the Apple menu. If you are trying to reboot into a special mode because your iMac is locked up, shut down the computer by holding the power key for five seconds.
Press the power key briefly while holding down the Option key. This provides a list of available disks with bootable Mac systems. Click on an alternative startup disk to boot your Mac from that drive, such as an optical disc or an external drive.
Repeat Step 1 if this does not produce the desired result.
Hold down "S" and press the power key to launch into single-user or "safe" mode. This provides you with a Unix prompt and instructions you can use to repair your boot volumes. These commands perform similar repairs to what can be done within the Disk Utility application, but this process will allow you to repair an iMac that cannot be booted.
Tips & Warnings
- Unlike older Macs running Open Firmware, Intel Macs do not provide a command line to work with the EFI boot system. If your iMac is running Mac OS X 10.7 Lion or later, you can also hold down Command-R while starting your Mac; this launches the special recovery partition that is installed on every Mac running Lion or later operating systems.