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Sometimes in Debian Wheezy happens when you get latest update which contains new kernel header, GRUB automatically updates itself. This might arise problem if any other operating system installed alongside with Debian and if at the time of auto updating related partition to that OS is not mounted. On such condition, GRUB automatically removes other boot options and when you reboot your system, you will realize that boot option to other OS is missed. The reason that GRUB removes automatically other option(s) can be found in the ability of GRUB that automatically detects failure boot options and remove them and since related partition is not added GRUB considers it as failed boot.
Now in order to overcome with this problem one simple solution exists. You just need to mount other OS partition and also update your GRUB once after you reboot the system. Make sure that os-prober package is installed on your system. You can install the package with following command,
$ sudo apt-get install os-prober
After that you need to update your GRUB with update-grub command like below,
$ sudo update-grub2
And finally reboot your system to make sure that other OS boot option(s) back.
For more information please refer to following links,
