|GNOME Disks writing Ubuntu ISO to USB stick|
Other than Ubuntu Startup Disk Creator and unetbootin, there's a third option available for creating a bootable USB stick. The best part is it's built right into your Ubuntu installation just like Ubuntu Startup Disk Creator so no need to install additional tools like with unetbootin.
A quick way to access the feature is to right click on a Ubuntu ISO you've downloaded and select Open With > Disk Image Writer. That'll bring GNOME Disk's restore disk image tool right up and let you select your USB stick as the target.
Once done you're ready to go. It's as simple as that.
The downside is this overrides your USB stick's partition unlike Startup Disk Creator or unetbootin so be sure to backup anything that you want to keep before proceeding. You'll, of course, need to reformat it for use after you're done unless you want to stick with the tiny partition size.