I personally wouldn't know the exact details of it, because I used a LiveCD, but I don't think Mint will go directly onto a USB drive for some reason. If you check over at persistent pendrive Linux, they have some sort of fix that requires you to use a Windows-based machine to run a .bat file in order to prepare Mint for USB booting.
If you can prove me wrong (and chances are someone probably can, because I'm a techno-retard most days), I guess in order to eliminate any variables, check the md5 sum of your ISO file you used originally with UNetBootIn. Sometimes if you torrent the ISO as opposed to downloading it directly or through a mirror, the fact that the ISO is split up into so many parts can cause t to become corrupted when it comes back together.
"People should better think of their computing devices as facilities lended by the DHS." -Berke Durak, Linux wipe (secure file deletion) development.