The usual cause of firmware not being accepted when using a thumb drive is that the .bin file is not located in the root of the drive (main directory) and renamed properly to update. Rename the file to update and try again.
Make sure the correct firmware is being used for the model video recorder being flashed. If the firmware build date is over a year old, then updating to an intermediate firmware version may allow for the most recent firmware to be flashed.
Reformat the thumb drive to either NTFS or FAT32 and try flashing the firmware again. Try inserting the thumb drive in the other USB port on the video recorder.
If all else has failed, contact DH Vision technical support for assistance.