Ubuntu 12.10 doesn't boot

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http://askubuntu.com – I just installed Ubuntu 12.10 onto my iMac. I am using rEFIt to boot ubuntu. When I choose the ubuntu option from the grub boot loader Ubuntu does not boot. My specs are: Intel core i5 quad core 2.5 ghz 4 GB of ram AMD Radeon 6750M with 512mb of VRAM IF you need some more information just ask and I will give it to you. (HowTos)