[SOLVED] Trouble with GRUB re-installation

[gold_finger]

a) do I type everything you wrote me in my terminal (including "for i", "do", "done", "exit"

Yes.  Do all lines (one at a time) exactly as shown.  (Just need to change /dev/sdXY to whatever your LL root partition is; and /dev/sdX to whatever correct drive letter is.  If not sure which is root partition, run sudo blkid -c /dev/null and look for it in the output.

b) don't I have to mount back all of the partitions one by one?

First command mounts the root partition to the "live" DVD/USB environment.  Last command unmounts it when you're done re-installing grub.  After that shutdown computer and remove live DVD.  Reboot and your installed LL on the hard drive should now boot properly.  If you've got other distros on the drive, they will show-up on the grub boot menu too.

No need to remount anything in live environment if that is what you were asking.  No need to mount any partitions other than the LL root partition to the live environment to repair grub booting.  Only exception to that would be if you had a separate "/boot" partition; but I'm assuming that's not the case because you didn't say you had one.  (If you didn't explicitly create one when you first installed LL, then you don't have one and the boot files are on the root partition.)