Is that 500gig file one single video file? That sounds extraordinarily large for a single clip.
Do you know what setting the DNxHD is?
Because DNxHD 120, 23.98fps, 1920×1080, you would be looking at about 10 hours to fill 500gb! (I’m getting this figure from the Formats app, by the way, so don’t know how accurate it is: https://grayhourmedia.com/formats/)
If that’s the case, I would definitely think about two things:
1. Breaking the clip down into smaller chunks (maybe an hour or so each)
2. Making lower res/bitrate proxy files to work from