ElementaryOS does not boot from USB

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http://unix.stackexchange.com – I downloaded Elementary from the official site. No problem with the iso. I checked the md5sum. I used unetbootin to create a live pen. I have a pen formated with W95 FAT32, with a lot of mp3. But I have enough free space. Well, after the boot, when I choose Try without install I can read /init: line 7: can't open /dev/sr0: No medium found I read something about disable floppy disk on setup but I don't have floppy drive. I also tried to adjust to boot from the iso directly from the HD, but it was too difficulty for me. No progress. Any help? (HowTos)