Arrghh...So frustrated with this problem.
Today I downloaded the most recent Ubuntu, being completely new to Linux. My plan is to run it on a partitioned drive with Windows 7 on a separate SSD.
Anyway I downloaded Ubuntu 13.10 ISO and THEN downloaded the Pen Drive Linux USB Installer. After using that to format my USB drive properly and then put the ISO on the drive, I restarted and entered the boot menu via the POST screen. I selected USB-HDD and it restarted as usual. But after POST it came up with the message "Missing Operating System" on a black screen. I have also tried putting the flash drive to highest boot priority in the BIOS, to no avail.
So does anyone have a solution to this problem? I've seen many questions on this before but nothing helps.