I've googled and searched the forums, and haven't found anything that has helped with this. (Copied and pasted from AskUbuntu, where I've gotten no replies).

I have a late 2012 Mac Mini and I've successfully installed Ubuntu 13.10 on a partition on an external hard drive. However, I cannot boot it during start up. I've booted with a rEFInd CD in the drive and it showed the Linux partition but when I selected to boot Linux, it froze on the gray screen with the penguin and I had to restart the computer.

Is there something I'm doing wrong with rEFInd? I think it might have something to do with where I installed the bootloader for Ubuntu. I was unsure, and I did some research and installed it to what I thought was the right location, but I could have screwed it up. I installed it on my external drive (can't remember exactly where now, but I think it was on the root partition). Any ideas, suggestions, and help are greatly appreciated.

Maybe, but you won't know until you can actually boot it. Not sure what 'rEFInd', some software to modify the Mac bootloader? There are some members here who use a Mac who might be able to help. You might give some specifics on what you did. I've never used a Mac but it you did install the Ubuntu bootloader to the / (root) partition of its filesystem, you would have to modify the Mac bootloader somehow to have it poiint to it. If you've done that, some specific steps on exactly what might help someone point you in the right direction.

Installing Ubuntu on an external in a case like yours would probably work better if you had installed the Ubuntu bootloader to the master boot record of the external. You can easily do that. You haven't given much information on what else if anything, is on the external and what your intentions are.

From rEFInd's website: "rEFInd is a boot manager, meaning that it presents a menu of options to the user when the computer first starts up, as shown below. rEFInd is not a boot loader, which is a program that loads an OS kernel and hands off control to it." Everything I read about dual booting OS X and Linux said that rEFInd was needed in order to have the Mac see Linux and be able to boot from it.

How do I install the bootloader to the master boot record of my external? Can it be done without reinstalling Ubuntu? I partitioned my external HDD as a GUID partition table. It is 1 TB, with a 500 GB partition for Time Machine backups for the Mac, a 300 GB partition for general storage, and a 200 GB partition for Ubuntu. The Ubuntu partition is actually the first partition on the drive if that matters. My intentions are just to be able to dual boot Linux from my external drive rather than partitioning and taking up space on the Mac drive.

Read the information at the Ubuntu forums link below. It provides several methods for doing that. The best method in your situation is going to be installing from the Ubuntu Live CD which you used to install it. See Section 2.2.3 in the lin below:

Yes, again see the link below.

https://help.ubuntu.com/community/Grub2/Installing

I would suggest reading the entire page to familiarize yourself. Section 1.1.4 has some info on BIOS/GPT.