USB flash disk-nothing on it, but says its full

I transferred some pictures from my family's PC to my mac and then deleted everything on the flash disk.
Now when I go to put something onto it, it says the disk is full.
HELP?
 
Tried formatting it? Right-click it and click format. In the window that pops up, click the Format button and once it's finished, close it and try to add files to it again.
 
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