Comment 4 for bug 1630017

Oh, indeed. I hadn't seen it initially.

sdj1 shouldn't have any kind of particular filesystem, so I'm not sure what grub-install is trying to check there (I will look in the code now), or if it's directly because the root is formatted in jfs.

In the meantime, when this fails could you try running the following command in a shell? You can access the shell from the debian-installer main menu after the failure.

chroot /target grub-install -v --force "/dev/sdj1"