Originally Posted by williepabon
In fact, the drive doesn't show up on the Start Up Manager while holding the Option key. Another thing I did was, I connected the drive after boot up and the system reports the message: "The disk you inserted was not readable by this computer."
I installed Ubuntu on the external (usb) hard drive using my Windows pc with the 12.04 iso I burned on a CD.
Disk will not be readable because MacOS cannot read ext file system Ubuntu uses by default, but it should still show up in the boot manager.
You may need to take a look at rEFIt. http://refit.sourceforge.net/
to get your external drive to show up. Before you do, try a different USB port on the mini. Also try plugging it in while you're already in the boot selection menu.