I've had this problem since at least Lion, perhaps before, when I stick a USB in with mp3's for example (could be any file type) and open the USB stick in Finder and delete a few mp3's, they appear to be gone as in I can no longer see them on the stick in Finder but when I plug the USB stick into my media player the old files are still there! This also affects the space on the USB stick as in when I delete files I do not recover the space...the only way is to completely reformat the drive...what gives?