Lots of post mentioned that before. I recently reinstalled Ubuntu and I had three drives in my computer. After installation, I wanted to remove one of the drive since it appears that the system was extremely slow and unstable with all three drives plugged in. I suspect that it was a power problem since I had a power hungry NVidia video card installed also.
Unfortunately, GRUB was not installed in the “right drive” but exactly the drive I wanted to remove. Anyway, the command to save the day is
dpkg-reconfigure grub-pc
Just run it after getting into Ubuntu.