Comment 26 for bug 1117292

FWIW - I also had trouble with this on 14.04 LTS.

I did 2 things, the latter of which I think is the reason I was able install:

1. Recreated the USB using the startup-disk-creator in Ubuntu itself (not Unetbootin!)

2. Plugged the USB stick into a USB 2 (black) slot, as opposed to USB 3 (blue)