This solution requires installing linux. If you don't want to install linux see this gist: https://gist.github.com/blackgate/17ac402e35d2f7e0f1c9708db3dc7a44
Disable SIP
If you can boot into recovery mode (Command + r), open the Terminal and enter:
csrutil disable
If you can’t, create a USB stick with rEFInd as described here: http://www.rodsbooks.com/refind/sip.html#disable_in_refind
Boot into single user mode (Command + s) and change the gpu-power-prefs nvram variable:
sudo nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00
reboot
Install ubuntu alongside with macos following the steps in the description of this video: https://www.youtube.com/watch?v=ELUzf0MDQS0
After following those steps, when grub appears press 'c' and execute:
ls -l
Annotate the uuid for the macos partition
Press ESC and boot ubuntu
Open the terminal and edit the custom grub entries
sudo nano /etc/grub.d/40_custom
Add the following entry to the end, replacing the <ANNOTATED UUID> with yours
menuentry "macOS" {
insmod hfsplus
outb 0x728 1
outb 0x710 2
outb 0x740 2
outb 0x750 0
search --no-floppy --fs-uuid --set=root <ANNOTATED UUID>
chainloader /System/Library/CoreServices/boot.efi
}
Save the file, update grub and reboot
sudo update-grub
reboot
You should now be able to boot macos with AMD graphics disabled.
When you update macOS and you are not booting into grub anymore, you can use the rEFInd usb stick created earlier to boot into grub, and start macOS from there. Then on macos Terminal execute:
cd /Volumes
sudo mkdir efi
sudo mount -t msdos /dev/disk0s1 /Volumes/efi
sudo bless --folder=/Volumes/efi --file=/Volumes/efi/EFI/ubuntu/grubx64.efi --setBoot
sudo bless --mount=/Volumes/efi --file=/Volumes/efi/EFI/ubuntu/grubx64.efi --setBoot
And it should be working again.
Ok thanks. Just have the one HDD so guess it is just a glitch. Thanks for your input and expertise.