I dont know what unetbootin is, but I usually use DD to transfer the image.
this sounds like a problem which might be caused by some "intelligent" software while applying the image to usb drive.
the iso contains a partition table, so you simply may put it on any harddisk like media.
just do a
dd if=<iso-image> of=/dev/sdb bs=1M
and be sure to check if /dev/sdb is your usb drive. else you could wipe your hardrive.
best to check which one is your usb drive is to mount it, then issue "mount" which lists all mounted drives.
one of these should be your usb drive.
before using dd, you should unmount it first.