I have installed Ubuntu 12.04.1 LTS on an external drive from a CD but it will not boot. iMac 2011, OSX 10.7.4

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http://askubuntu.com – I have followed all installation instructions and partitioned the drive as follows: 475 GB for Ubuntu and its files 25 GB of swap space 5 MB of reserved BIOS boot space I attempted placing the bootloader in the BIOS boot space, in the Ubuntu partition, and on the drive itself - none worked. When I tried to boot Ubuntu, it said that the drive was not bootable and asked for a boot disk. When I put one in, it gave the default screen asking if I wanted to try Ubuntu or install. Did I do anything wrong? Any help is much appreciated; I'm getting really tired of OSX and love the look and feel (HowTos)