...Would like to boot an UBUNTU image.
but, even if i made all procedures according ubuntu specific (http://www.ubuntu.com/download/help/create-a-usb-stick-on-mac-osx),
my usb pen is not seen in the boot phase...
Since that link encourages you to use the DVD approach instead, does it boot if you try that?
In any case, you might find it a lot more convenient to run Ubuntu (or any other Linux distro) using a Virtual Machine in, for example, VMware Fusion.
no good result:
if I can't see the ubuntu usb pen when booting, i can't see the same usb pen in System Preferences > Startup Disk and restarting, as you suggested.
Any way, now it's a obsession.
I don't care Ubuntu System, but it means that i can't boot any kind of external usb.
I tried the same operation with an apple MacBook Pro and the result was positive.
reading a few in internet, i see that the matter is about the EFI driver version:
MacBook pro has a different drive version compared to iMac 27
I have the iMac (27-inch, Mid 2011) type.
Macs detect bootable Linux USB disks as Windows drives on Startup Manager (pressing Option key when your Mac starts) and OS X. Only some Macs supports the Windows installation through an USB disk, so maybe that's preventing you to install Ubuntu. Burn a DVD with the same Ubuntu ISO and press C key when your Mac starts to boot from this DVD