Grub failling instalation on Old Macbook 4,1 (2008)

You also could try rEFInd:

See... this is why I need to read the Whole O.P. before I rush to answer...

Here is a Full-Length guide that you can peruse to see if it includes any steps you might have missed. This may be a more direct option than guessing in the dark:

You might consider a Fresh Start while following the guide as well.

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