Problem: On iMac 2009, Ubuntu 20.04 can boot from USB in safe graphics mode, but after installation it only shows black screen

To fix, this, one has to change the configuration of the grub bootloader on disk

Start Ubuntu in safe graphics mode from USB, then mount the disk

sudo mount /dev/sda2 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt

Then edit `/etc/default/grub` using nano

Modify

GRUB_CMDLINE_LINUX=""

to

GRUB_CMDLINE_LINUX=“nomodeset”

and update the grub bootloader using

update-grub

and restart from disk.