The problem is well known; if you have an installed openSUSE 11.4, you can use it's own USB-installer, 12.1 I don't know.
There are many different UNetbootins - I always use PartedMagic/ UNetbootin, the best of them,
but with openSuse it might be a problem.
With Therminal command "dd". I did it in the past with success, but at the moment i can't say how to proceed.
I'll search in the net and post again.
chapter 17 and 19 (Mint!)