I have a MacBook Air with 256 Gb drive. I had 127 Gb left. I edited two HD videos on just over an hour each using iMovie, the output the result to a file (each final file was 10 Gb). That left me with 17 Gb of Drive space left. I offloaded the two final files (-20 Gb) then deleted both projects, and found the original files and deleted those too. Emptied the bin, and I now have 74 Gb of Drive space left. Where did the other 50 Gb go and how do I get it back?!?