Where did you buy it from?
Are you using the drive that came with it? I still haven't been able to account for all of the variance in flash drives, but the ones we ship with the unit are the ones that have had the most compatibility testing.
Make sure to start up the unit with the drive not inserted, then insert after the bootloader screen appears.
I just helped someone else out that had a similar problem and it suddenly starting working after he wrote settings from his GT to the flash drive, then rebooting into the bootloader. I don't know if that was a coincidence or not, but it's worth a try.