You got it direct from the EXACT same motherboards's support site?
I found one that had an "F" at the end so I think this is correct. THIS is the name of the latest BIOS:
"ROG-STRIX-X370-F-GAMING-ASUS-0813.CAP"
*The best answer I could find suggests the FAT32 format isn't properly done. Try redoing it using the FULL FORMAT option. I've also had weird issues in the past where my USB stick went wonky and I fixed them every time by using THIS TOOL:
HDD Low Level Format Tool
http://hddguru.com/software/HDD-LLF-Low-Level-Format-Tool/
use the "free" option when starting, then do the LLF (don't choose "quick") which will take a while depending on USB size/speed. Then redo the FAT32 format.