Hi Matt,
We assume you have installed RadarBox to your program files directory. If so if you go to Program Files\AirNav Systems\RadarBox\Data you should see quite a few files. You should see a MyLog file which you can copy to your D drive.
You might want to copy the whole data folder instead.
You can then install RadarBox to your D drive and copy the data folder to your D drive directory and it will continue to save it there.