How to make GRUB go again after an event (hard drive replacement, Windows pissing on the MBR, etc.):
1. Mount (or cause to be mounted) the target root filesystem.
2. Become root enough (if you're not there already) to issue this command: grub-install --root-directory=/media/thenewdisk '(hd0)'
(where /media/thenewdisk is the root of the target
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