To Control the Creation of Automatic Drawing Backup (.bak) Files
By default, the program creates a [drawing file name].bak file when you save a drawing. When you save, the current drawing overwrites the previous version. That previous version is saved as the backup file, should you need to restore the drawing.
Right-click the drawing area and choose Options.
On the Open and Save tab, under the File Safety Precautions heading, select the Create Backup Copy With Each Save check box.