Heh. RB is all over that and almost certainly the problem.
But, I've put Fam14 OS or Game loaders on flash drives on a Win10 system where it does create those hidden files that will always break a diag flash drive for sure.
But for simple file drives, I've had no problem at all even if it did have hidden folders/files on it with flash drives used to load the OS or Games. As long as it was FAT32 formatted.
My bet is the flash drive used is either exFAT or NTFS formatted. Which Fam14 cannot read.

Once formatted as FAT32 (you "may" need to use a utility like fat32format.exe to do so, which can be had on the web pretty easily). But then when game loaders are placed on it, they should show up normally and can be loaded.