I have tried uunetbootin and mkusb, to try to install Ubuntu 18.0.3 onto a usb, as an OS, not as an install disk.
With uunetbootin, it just over-writes the Ubuntu on my laptop, and installs an 'install' disk onto the usb, even when i select only the usb.
With mkusb, it just freezes when I choose the usb as the target disk to install to, with persistence, and choose the live install dvd of 18.04.3 in the dvd drive as the source disk.
The only way I have been able to do it, is by locking the hard-drive in the bios (intentionally entering the wrong password on start-up), and then F12 booting from the Install DVD. Then when Ubuntu loads, clicking on install, and choosing the usb as the target device.
Ubuntu gives an error message of being unable to write/close the hard-drive, which I just click 'ignore', and after 3 times of this message, it installs a perfect OS with persistence onto the usb.
Works for me on a Dell laptop, but surely there must be a easier,simpler way ??
Thanks,
vedaal