On Sat, 24 Apr 2021, Chris Murphy wrote:

> You need to use -B to bind mount the pseudo filesystems, and probably also
> need to include /sys

> grub2-install /dev/sda

mount /dev/sda3 /mnt/a3       # already done
mount -B /proc  /mnt/a3/proc
mount -B /dev   /mnt/a3/dev
mount -B /dev/pts  /mnt/a3/dev/pts
mount -B /sys  /mnt/a3/sys

chroot /mnt/a3

grub2-install /dev/sda
mount /var                 # grub2-mkconfig wants it
grub2-mkconfig > /boot/grub2/grub.cfg
exit  # out of chroot?

reboot