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Puredyne/Repair grub2
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It can happen that you're stuck at grub prompt with no boot possible (after re-installing Windows, or randomly on a mac).
There are several things to try to repair Grub:
boot on a liveCD/DVD and re-install Grub:
detailed tutorials here: Grub2LiveCdInstallGuide or here: https://help.ubuntu.com/community/Grub2#Reinstalling%20GRUB%202
SuperGRUB2Disk:
on a mac, if you (accidentally) installed Grub2 on the hard drive MBR and ended up with 2 linux icons (Boot linux from EFI and Boot linux from partition X) and none of them boot anymore.
Booting on the SuperGRUB2Disk repairs the MBR grub install.
note: after this, in the Refit menu, you might have to boot linux from EFI and not linux from partition X anymore