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If you have damaged the boot sector of one of your partitions (e.g. by installing GRUB in it by mistake), you may have troubles, eg if it's a Windows partition you may not be able to boot into Windows any more.
Here is how to repair it:
1) Run Ubuntu (works also in live-CD)
2) Install and run Testdisk, by activating the Universe repository (in your Software Sources) and typing in a terminal (Ctrl+Alt+T) the following command:
3) Via the arrows and the Enter key, go to the [No log] menu,Code:sudo apt-get install -y testdisk && sudo testdisk
4) then select the disk where the broken partition is,
5) then select [Proceed],
6) then choose the type of partition (generally [Intel]),
8.) then select the Windows partition with [Boot], it will display something like :
9) Check that you have "Status ok" below "Backup boot sector"Code:Boot sector Status: Bad Backup boot sector Status: OK Sectors are not identical. A valid NTFS Boot sector must be present in order to access any data; even if the partition is not bootable. [ List ] [Backup BS] [Rebuild BS] [ Dump ]
10) select [Backup BS].
Done. Now you just have to reinstall GRUB into the MBR of the disk, e.g. by running Boot-Repair and clicking the "Recommended repair" button.