Comment 123 for bug 1355698

Re #122
I can tell you why.
Unetbootin only accepts a USB-pen with a FAT32 partition thereby ensuring that you can boot from a UEFI system.
If you USB-pen hasn't got a FAT32 partition you'll have to create it yourself, or it won't accept you pen.

If you use the dd command ( or gnome-disks for that matter ) you don't have to care about partitions because these tools just dumps the raw disk image on the USB-pen no matter what content was on it before.

But there is something wrong with the iso from elementary , it doesn't automatically create a 2,4MB FAT boot partition like Ubuntu or Mint does when you use dd ( though it seems to be present on the iso ), thats why it's essential that you use a tool like Unetbootin if you have a UEFI system, because a UEFI system requires a FAT32 boot partition to boot from USB.