Flash Drive Confusion

Ok so I need to do some major maintinence on my computer so first I wanted to back up any thing important. So I have a 256GB flash drive that I started to upload all my video footage to. After spending a lot of time uploading a lot of video I checked some of the files and several said empty folder. I assumed I screwed up and uploaded the files again. This time I checked and they were there. I uploaded a few more and I noticed that one of the files I reuploaded and checked before again said empty folder. Now I am noticing that The files that show up properly saved add up to roughly 50gb and the computer is telling me that there is still roughly 50gb free. That means that I'm missing 140gb of space. Nothing is showing up when I look at the drive so how do I find out what's taking up that space and how do I free it up?Also any idea how files that were there one minute are suddenly gone another?