What to do when grub-install fails

I was trying to reinstall GRUB to use the new kernel image I compiled, but it was failing:

# grub-install /dev/sda

Fortunately using the “grub” terminal solved it:

# grub
grub> root (hd0,0)
root (hd0,0)
 Filesystem type is ext2fs, partition type 0x83
grub> setup (hd0)
setup (hd0)
...
Done.