This was addressed in the Grub solution discussion. High Sierra has that brightness issue regardless if you're using the EFI or GRUB fix (if I am not mistaken). The best thing for you to do is downgrade to Sierra, since there's nothing really new with High Sierra anyway. --- Post Merged, Jan 17, 2018 --- Try this first: Boot into Single User Recovery Mode (Cmd-S-R) or Single User Mode (Cmd-S) and fix the disk via command line. http://www.everythingmacintosh.com/tech-notes/repair-your-hard-disk-in-single-user-mode/ This should repair/rebuild the hard disk if the only problem is the OS. If the repair fails, then consider reinstalling the OS. If you can't reinstall the OS, it's probably the hard drive.