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 speaking of tools, for use with isos, is there anything that's supplanted YUMI as a "I want to make a single flash drive able to boot six different things" tool? I've been out of the loop ever since my last job switched to PXE and clouds, given I also don't install OSes for fun these days.
@r4v5 I don't really know, I never used YUMI. It's not the kind of things I'd use, I really prefer a raw dd, because ISOs go out of date quickly.