I know this thread is a bit old, but am posting to help others who are looking for help.
So, I had the identical error, except I was creating a multiboot USB drive with several bootable OS ISOs copied by YUMI. I have a SanDisk 16GB USB3 USB drive.
By following the instructions earlier posted in this thread, I was able to use DiskPart in a CMD window to properly format with FAT32 and set this partition as Active. After adding the ISOs through YUMI again I was able to successfully boot in to the boot loader.
I believe the root issue in this entire thread is that the said USB drive does not have an ACTIVE partition, which I believe will NOT be bootable. I think this because before I formatted with DiskPart, I viewed the details of the partition I was trying to boot from, and it was NOT ACTIVE.
Hope this helps someone...