[ubuntu] Grub Rescue Error- Help!

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http://ubuntuforums.org – Hi, I'm a pretty new Ubuntu user, and I can't figure out what to do from searching other threads. Anyway, here's my situation: I had Ubuntu 12.04 installed on a partition, and OSX 10.7 on another, and it worked great. Then after I installed 10.8, my Ubuntu partition no longer worked. It gave me: "error-unknown filesystem grub-rescue>" After doing some reading, I tried to repair grub by booting from a Live USB disk. But now I'm getting the same "grub rescue" prompt when I try to boot from the live usb! Any ideas? Thanks a lot (Hardware)