The main point of my post was that you should unmount /boot (sda1) before running tune2fs. But, as an experiment - I was able to remove metadata_csum_seed from a mounted ext4 filesystem using tune2fs (I would still unmount the filesystem first for safety).

Having re-read the thread i agree with colorpurple21859 - you need to upgrade e2fsprogs on the LFS system. Or maybe easiest, run tune2fs from the *host* Archlinux system to remove the filesystem feature on sda1. Then, enter chroot and install grub.

lfs 11.3 has E2fsprogs-1.47.0

Hi colorpurple21859, Good news! As expected, the msdos partition table changed nothing, and I met the same obstacle.

However I opened a second terminal, as ordinary root, ran the two tune2fs commands from there,

# tune2fs -l /dev/sda1 | grep metadata_csum_seed
# tune2fs -O ^metadata_csum_seed /dev/sda1

then went back to the lfs working shell and repeated

grub-install /dev/sda

and ... da dah! ... all went as it should, and the installation boots fine. So I'm very pleased. Thanks to bryan_S also, for pointing out I needed to run those commands on an 'unmounted' filesystem, which I gambled would run from a shell on the Host System.

Now to mark this problem SOLVED. Thanks to all