Sounds like you're using a mac pc, so I can honestly tell you I know next to nothing about your OS seeing as South Africa is mostly Microsoft and Linux reliant, but I can try to give you some general tips.
Usually what you have means that there is a virus on your USB stick/flash drive. It takes up all available space on the drive and then hides itself. In windows you would simply select the options to reveal all hidden and system files and folders and then delete the offending virus files.
I don't know whether mac has an option like that, but it should if it's worth anything.