best contact evga and see ? I could see one pendrive not being compatible or detected but if you use a few different brands or what ever of pendrives you think one would be good and recognized and work properly .
is there any pressing reason you feel you need to flash ? unless you have a compatibility issue with new hardware it resolves or stability issues do to that its not recommended to flash anyway just for the sake of flashing [in other words don't fix what aint broken ]
I got 2 old boards still on there first release bios never needed to flash then at all to this day even with new hardware
now from here shows a extra steps ??
''•First go into your BIOS and load defaults, save and exit
https://forums.evga.com/EVGA-X99-ClassifiedFTWMicro-BIOS-112-Released-m2254106.aspx
need to contact evga and let them spell it all out on proper flashing steps then blame them if it fails