USB Became read only mid-copy?

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http://www.linuxquestions.org – I copied 10 videos to a 64gb flash drive fine. Everything copied over fine. Had another set of 10 videos I needed to copy and after that. Started copying the second batch of ten and after the first video (and probably half way through the second) it stopped and said 'Error opening file '/media/micah/USB20FD/(the video)': Read-only file system'. How can this happen when it's half way through copying a file? (HowTos)