How to Force iMac to Boot EFI

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.
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