It is highly suggested that when doing firmware updates or the Firmware Recovery process through USB flash drives that you complete this process with a Sandisk drive as mentioned on the below knowledge base article. Not all flash drives will be able to complete this process correctly, and the suggested drives have been tested well to complete this process:
NOTE to Mac Users - If your Mac is running OS X 10.11 El Capitan or later, you will need to make sure to adhere to the following extra step in order to format a USB drive for loading firmware on your mixer (Users with Mac running OS X versions earlier than 10.11 can skip this):
Leave name set to "Untitled" & set Format to "MS-DOS (FAT)"
Change Scheme type to "Master Boot Record"
After verifying your settings look like the screenshot below, click "Erase"