To play back uncompressed video, Pro Tools copies the black file to the drive that the uncompressed video if being played back on. This is a fix for -6042 errors that showed up in extreme cases when playing uncompressed video clips from clip to black to clip and the boot drive (where the black files normally reside) is not as high performance as needed).If the video file(s) live on a Unity volume, that workspace must be write enabled so that the black file can be written back, otherwise Pro Tools will crash.