You shouldn't be using Unetbootin anymore since years ago.
This piece of software comes from a time where .iso images of Linux distros were made solely for CDs and didn't run on other supports. Unetbootin did replace the bootloader on these images with one that would be compatible with USB drives and such.
Now, ISOs are compatible with nearly any storage type, and the Unetbootin replacement tend to break stuff on the distro installers.
Alternatives ?
Windows : Rufus
Linux : Google "dd iso usb"
@Siphonay no one can claim to linux unless they ran oldschool slackware and compiled their own kernals and damned near every fucking binary. Pfft.