I'm on a mac and have tried installing Ubuntu 13.04 on my external hard drive (which also happens to be my time machine backup drive). I created a live USB and used it to install Ubuntu on the external hard drive. I formatted my linux partition to ext4 and I created an 8gb swap partition as well.

After going through the install process it said it had installed and needed to restart. After I clicked restart it got hung up on a black screen and after 20 mins I just hard shutdown the computer. When I restarted and held down the option key it doesn't show a bootable linux drive. All it shows is my hard drive, the recovery drive, and the time machine backup partition on my external hard drive. It doesn't show the linux partition that I just installed Ubuntu on.

What is going on? Is there something I missed?