How to Reinstall Ubuntu Grub2 Bootloader after windows wipes it out
1. Use Ubuntu 9.10 live CD (you can download it at Ubuntu.com) to reboot your PC
2. Mount the harddisk partition where Ubuntu is installed.
3. Find out the correct directory and drive name:
mount | tail -1
You will see output similar to this:
/dev/sda6 on /media/0d104aff-ec8c-44c8-b811-92b993823444 type ext4 (rw,nosuid,nodev,uhelper=dev
4. To reinstall GRUB by specifying the correct directory and the correct drive name:
sudo grub-install --root-directory=/media/0d104aff-ec8c-44c8-b811-92b993823444 /dev/sda
5. Reboot from Hard disk (remove the live cd)
The above procedure will restore your old grub, but if you have installed windows 7, you need to upgrade your grub:
sudo update-grub
For Ubuntu 9.04 or Grub 1, please see this page:
http://thanhsiang.org/faqing/node/104
also interesting!
Boot Problems:Core On Windows
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Symptoms
Windows might not boot.
The file RESULTS.txt generated by the Boot Info Script shows /boot/grub/core.img among the boot files of the Windows system partition.
"update-grub" does not detect Windows and displays the message :
ls: cannot access [some/path]/boot
Boot: No such file or directory
Booted into Windows, you have two folders both named "\Boot" and with the same contents.
Cause
Grub2 was installed with Windows system partition chosen as the root-directory. This causes the folder /boot/grub to be created on the Windows system partition. Since ntfs partitions are case insensitive this leads to confusions between "/boot" and the already existing folder "/Boot"
Solution
Boot into your Linux OS and delete or rename the folder /boot on the Windows system partition. Make sure that your don't delete the /Boot folder. The /Boot folder contains the file "bcd" which is necessary to boot Windows Vista/7.
found via google