Recover GRUB on Debian after (re)installing Windows
Boot knoppix and start the root console.
Type:
grub
root (hd0,0)
setup (hd0)
quit
Reboot
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If you use Ubuntu
sudo -i -u root
grub
find /boot/grub/stage1 (remember the output)
root (hdx,x)
setup (hd0)
quit
Reboot