Did you make sure the usb drive was above your hard drive in the boot order in your bios? Alternatively, many let you press a key (usually F11 I think) to select the which device to boot.
Something else to watch out for (not the problem you're having now) is that unetbootlin seems to have trouble copying the server/alternative cd correctly to usb, due to this:
https://bugs.launchpad.net/unetbootin/+bug/373089
I gave up and used the cd for Lucid server installs.
A third option I've tried is installing within virtualbox, then migrating it to the partition, but that presented other difficulties, like vboxmanage -clonehd having an undisablable feature where it changes the partition uuids, but there's ways to fix it, and other ways to copy it.
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