i recently installed three ubuntu flavors.
i had problems with unetbootin; i solved it by making the usb's thus: stick the usb in. does it show up in your file manager? then open a terminal and type 'mount' without any arguments. here, it says:
so /dev/sdc1 is my usb stick. make 100% sure! then type:
/dev/sdc1 on /media/me/12fe51dd-0a7b-43f4-878d-35241aff5db6 type ext4 (rw,nosuid,nodev,uhelper=udisks2)
where sdX is your usb. 'of=/dev/sdc' would be the right value, not sdc1.
sudo dd if=/path/to/your/ubuntu.iso of=/dev/sdX
big fat warning:
dd (nickname disk destroyer) is a very powerful and potentially dangerous tool! if of= (output file) is set to a wrong value, data loss may occur!
maybe the sudo is not needed when copying to usb-stick.
also you may need to unmount the stick before executing dd (umount /dev/sdc1).