Comment 42 for bug 277903

I recently purchased a GA-990XA-UD3 and experienced this problem.
This is an UEFI board, with support for Legacy boot, etc.
I had no problems creating a TAILS persistent USB flash disk, within TAILS DVD. It booted normally from USB.

But then, when I used a Sandisk Cruzer Blade (another flash drive) to create a standard Ubuntu 15.04 Live USB, using the tool called "Universal Boot Installer", following the right steps. Everything in place, FAT32, etc, and to my surprise....

"Missing operating system".

I found out that the flash drive had two configurations, when investigating through gparted, and fdisk.

1) The flash drive was as GPT partition.
2) It had the msftdata flag.

I don't know WHICH one of those could have caused the problem.
I think this happened to this flash pendrive when I was playing around with "Windows 8 Disk Management" tools.

What I did was to completely wipe the flash drive, with gparted, recreating the partition table as msdos, and formatting as FAT32.
Using "Universal USB Installer" recreated the system, and then I found out it also flagged the drive as "boot, lba".

I hope this helps, if you're stuck.