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This update will allow your Mac to gain a prototype boot loader that was used on 2005 prototype EFI Macs. I found this boot menu left in the original iMac4,1 firmware and successfully ported it to the first generation MacBook Pro.
Use a USB device formatted as either HFS+ or FAT32, and copy the contents of the attached zip file to the root of your USB drive. The updating process will be done using an EFI shell on the USB + a tool called SPIUtil (EFIUpdaterApp without the safeguards).
To initiate this update, make sure your Mac is set to EFI Update Mode by holding down the power button until the LED rapidly blinks and you hear a 3-second long tone.
After that, when you boot into the EFI shell, type the following command:
Spiutil -s MBP11_0044_MODDED.rom
Do not unplug or power off your device in order to avoid a hard bricked Mac once the update process has begun. The device should restart automatically once flashing is complete.
In order to activate the boot menu, boot into the EFI Shell from this USB using the boot picker, then type exit. That will automatically take you to the boot menu where you can go from there :)
MBP11-Modified-EFI.zip (1.49 MiB / 1.57 MB)
Do not attempt to flash this EFI firmware on any other device besides the MacBookPro1,1, otherwise it will result in a hard bricked device.