Response to your question:
1. Make sure your script load successfully (if possible share your script here and then we figure out together). And if you we don't specific path of QVD then *.qvd file should stay at the same directory of your *.qvw
2. To save QVD in specific path we can use Store MyTable Into [D:\QlikView\MyTable.qvd];
3. Of cause the new data will override the old one.