It could possibly be that way before you exported the file, that you haven't ran a; chkdsk /f in command prompt for a while to check for corrupt sectors, so possibly as the HDD never got this check, your PC assumed that sector was fine to write to, hence a corrupt file. It's unlikely but is still possible. If this is the case then you might be encountering the start of a slow HDD death. Check your HDD Health with a good program of rating and see if i'm correct about corrupt sectors.