I have a 256gb USB flash drive that I want to use in my Mac Pro (early 2008, OS 10.9.5) to transport video files larger than 4gb. Disk Util reports 2 different formats - disk is MS-DOS (FAT), volume is ExFAT. Write status is Read Only. All options to erase and start over are greyed out. Does anyone know of a way to reformat? Thanks for any help!