Mac Pro 1,1 Mavericks, Yosemite, and El Capitan

Note for El Capitan users

El Capitan has a new protection mechanism SIP (System Integrity Protection). With this enabled (default) any operation that could previously be achieved by the root user (like replacing system files) is heavily restricted. This method will now work on a SIP-enabled El Capitan system, however you need to make some changes first...

You will need to exclude the boot.efi files from SIP, thus making them writable (overwrite) again.
To do this:

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