SYSLINUX problem in bootable usb

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It is a known bug. There will be a new and much more robust version of the Ubuntu Startup Disk Creator in the next version of Ubuntu, 16.04 LTS, which will be released in April. But now I suggest that you use another tool to make the USB drive an Ubuntu boot drive. I suggest that you try mkusb, according to this link

https://help.ubuntu.com/community/mkusb

You run standard Ubuntu, so you need an extra instruction to get the repository Universe. (Kubuntu, Lubuntu ... Xubuntu have the repository Universe activated automatically.)

sudo add-apt-repository universe  # only for standard Ubuntu

sudo add-apt-repository ppa:mkusb/ppa  # and press Enter
sudo apt-get update
sudo apt-get install mkusb

Installation/FromUSBStick