For a while there, I had a completely full USB flash drive that had no files visible on it. Why was it full when it looked like there wasn’t a single folder, photo, or video on it? Here’s the solution to this odd behaviour.
If that flash drive is full and you can’t put more stuff on it even if you delete files off it, try this. Plug in your flash drive, delete a file from it, and then empty the Trash on your Mac (click the wastebasket on the Dock, then choose Empty Trash near the top of the Trash window on the right-hand side). Voila, your flash drive will have empty space on it!