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I recall Ubuntu still does not boot that way.
Well, I just successfully booted my Mini 5,2 from the DVD amd64 iso image burnt on a DVD using my cheap Samsung external USB writer and apple's Disk Utility! No need for the expensive Apple USB drive! Booting is capricious however, I.e. sometimes Ubuntu boots sometimes it's just a black screen (must be a grub issue on ubuntu's side of things). Though it booted the first time (after I chose the penguin icon with a dvd showing in the rEFIt boot menu), on other tries it did not. The trick I found that made if boot was to choose the same icon but have a USB key with FreeBSD 9.0 installed on that key inserted in one if the usb slots of the Mini. Bizarre but true. Anyway, happily booting Ubuntu now on my Mini 2011, yeah!

Maximini
 
Ok. Ubuntu 12.04 amd64 DVD ISO was burnt to DVD. Putting it on the tray of my Samsung external USB DVD writer and closing it while in OSX Lion, it shows as unreadable by Lion. Fine click Ignore in the dialog box that appeared. Restart and choose the penguin icon with a DVD in rEFIt. If you have installed rEFIt and created free space at the end of your internal hard drive (25G appears enough) then you should be able to boot to the Ubuntu installer. You can choose to install alongside OSX or "them" (another choice proposed by the installer when you have several OS already). The install will complete with Grub2 auto-installed too. At restart, choose hhe penguin icon with a hard drive and Ubuntu should boot by itself. Easy. Sound worked at second reboot. Wired Ethernet works. Haven't tested wifi yet but it doesn't matter in my case.

Maximini
 
Linux on new mac mini

Okay, final word.

Followed the directions to install Ubuntu 12.04 on USB key from OSX using their amd64 dvd iso download (and converting that to a .img.dmg file as per their instructions then using dd to copy that on my USB stick). It worked flawlessly from the first try.

I tried that in the past with old USB keys. My new one a Patriot Rage 8GB worked when other USB keys did not.

This thread should be renamed as solved IMHO.

The only thing that irks me is that after using ubuntu my wireless Apple keyboard and trackpad have to be rediscovered by OSX.

I was even able to boot ubuntu on an external drive connected by FireWire.

Maximini
 
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