If it is rather in U-Boot (before GRUB starts) you can try a newer u-boot-rpi3 from Base:System:Staging project. (Just download and extract the .rpm, e.g., in File Roller to get the u-boot.bin and put it onto the FAT partition.) There's also an update to raspberrypi-firmware that just got accepted into Factory today. When using USB disks on the Pis always make sure they are externally powered.