Hi, vltcl!
Downloaded .iso is already partitioned and has bootloader installed into system area, so no need to complicate things. As it is device, not partition, image, it must be written to /dev/sdX, not to mounted /mnt/sdXY. So, connect Your USB drive, don't mount any of its existing partitions, or if they are automounted in Fedora, unmount them. Let's assume that Your USB drive will be recognized as /dev/sdc. Then
sudo dd if="path-to-Core.iso" of=/dev/sdc bs=1M
sync
Have a nice Core!