I've managed to install Ubuntu on a usb stick and made it bootable on a mac mini via the refit boot loader. I used a live cd to do this and also installed a ubuntu partition on the mac mini at the same time. Everything seemed to go fine. I reboot get my efi menu with four options, OSX, Linux on HD (mac mini hd icon), Linux on HD (usb hd icon) and Legacy OS. Choosing ether Linux works fine.

Testing the usb ubuntu installation on another mac, I installed refit boot loader and booted into it which was fine but the changes I made had not carried across (i left a txt file on the desktop). I thought this was persistence but it's not, if I mount the usb drive on the first mac booted into ubuntu on the hard drive I can see my text file in my profiles desktop on the usb drive so what seems to be happening is the first mac prefers to boot into the macs hard drive ubuntu installation even though I choose the usb one - what's going on here? It must be something small if I can boot fine on the second mac.