I am aware how old this thread is, but given someone with the same problem pointed me here when asking for advice, I thought I'd share my answer
The answer, alas, is to not use SDC, but the command line.
# (identify the flash drive - likely /dev/sdb or /dev/sdc or the likes)
# (you should be able to identify it by its size - or just remove the flash drive, run lsblk again, see what disappears)
sudo dd if=name_of_image.iso of=/dev/sdc # (as appropriate)
This will copy byte-for-byte the ISO to your flash drive, overwriting it completely. It will look like nothing is happening when you run the command, but that's just the 'dd' command - it's not verbose at all. Takes about 1-4 minutes depending on ISO side and write speed of the flash drive.
Note that if this is one of the official Ubuntu respins (Ubuntu, Lubuntu, Kubuntu, etc) then it should be fine to boot from flash disk. Other ISOs may not have compatibility for being written to and booted from a flash disk.
You can convert an ISO to "hybrid" (CD and USB compatibility) by installing the syslinux utility and running isohybrid (which will modify the ISO file itself - maybe do this on a copy of the ISO)
sudo apt install syslinux-utils