Ah, the Kobo Aura HD
PCB is 'E606C0'.
So it wouldn't surprise me if you had to rename /Volumes/KOBOeReader/.kobo/
upgrade/mx50-ntx/uImage-E606C0 to /Volumes/KOBOeReader/.kobo/
upgrade/mx50-ntx/uImage and move the original out of the way, to make the update boot image look nice and clean.