Grub on Mac via EFI (Linux: /dev/sda1 Darwin: /dev/disk0s1) (MacBook Pro Early 2011 if it matters)
I'm posting the solution that worked for me in case of anyone else that has this problem comes across this. I changed the path of the grub EFI executable from /efi/debian/grubx64.efi to /efi/boot/bootx64.efi. Now it is appearing as a boot device in the boot selector, but it still wasn't booting by default. So to make grub boot by default I ran the following command (from Linux, not OS X): efibootmgr -d /dev/DRIVE -p PARTITION NUMBER -c -L "Grub" -l /efi/boot/bootx64.efi, where "DRIVE" is my internal hard drive, with GRUB, OS X and Debian installed, and "PARTITION NUMBER" is the EFI partition ID, so for me it would be efibootmgr -d /dev/sda -p 1 -c -L "Grub" -l /efi/boot/bootx64.efi