A USB flash drive that will not format or is not recognized on the TD-25 may not meet the supported specifications. The TD-25 supports USB flash drives up to 32 GB in capacity, formatted as FAT32.
If the drive's current format is unknown, connect it to a computer and check its properties. If the computer allows reformatting, format the drive as FAT32, then insert it into the TD-25 and format it there as well.