There are two ways I think you can accomplish what you want. I would use the first option - purging (completely removing) G2 and then reinstalling it. Do this without your external devices connected. Make sure you have a reliable power source and Internet connection, as you will be left with no bootloader for a minute or so.
Run these commands. They will uninstall grub-pc and grub-common and then reinstall them. Due to dependencies, both packages do not have to be included in the commands. Make sure you install Grub2 to the drive (sda, etc) and not a partition.
Note: You can use TestDisk to write (clear) the MBR of Grub2. TestDisk is in the "universe" repository.
sudo apt-get purge grub-common
sudo apt-get install grub-pc
No Log > Continue > Proceed > Continue > Intel > MBR Code. You don't need to do this if you use the first procedure. Of course, if you do run TestDisk you will have to reinstall Grub2 once you have overwritten the MBR.