Make sure you are downloading a bios revision that was meant for the REVISION of your motherboard. For example, there could be G41MTES2L ver.1.0 and version 2.0 and version 3.0, and bios versions that work on one revision may not be suitable for use on another revision.
Next, try different USB flash drive media. Maybe it's a bad drive.
Also, make sure to unpack/unzip the bios image files onto the flash drive before you try to update. The bios cannot read the files if they are still packed in a compressed format like .zip or .rar, or .cab or .gtz.