Installing multiple ubuntu partitions on same machine without corrupting GRUB

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http://askubuntu.com – I had only ubuntu installed on my pc, but I felt like adding another ubuntu (same version) partition so I could have one just for work/study and another one just for personal generic use. I installed it and it worked out okay, but I found I needed more space on the new created Ubuntu partition, so I booted up from the Live CD, and used GParted to move/shrink/grow the partitions as the Ubuntu's tutorials suggested it was pretty straightforward. But in doing all this I messed up with Grub, so when I start my system it shows: error: unknow filesystem grub rescue> What would be the easiest way (HowTos)