I had a situation the other day where I had a 32GB USB Flash Drive that was "empty," but when I tried to copy a 3 GB file to the drive, I received an error that the drive was full or there was not enough space. How is this possible when there are no other files on the drive?
It has to do with the way MAC OS handles "trash." The other day I deleted 27 GB worth of files from the drive, and while your Trash bin remains full on the OS, the files are not actually deleted from your thumb drive, but rather moved to "hidden" or .Trashes status - hoarding all of your free storage.
There are a couple of ways to remedy this: Empty your Trash bin, manually delete the hidden files, or Format the drive (this will erase everything on it).
To manually delete the hidden files you may use the Terminal application or a 3rd party application to show these hidden files.
Click here to learn about showing hidden files from Terminal.
Click here to read about apps that show hidden files.
To Format your drive on MAC OS, follow these instructions.