Click on to highlight the dmg file then use get info command or pull down file menu>get info or show info, (depends on os10 version). Toggle the triangle for open with. You will see that os9 disk copy is choosen to open dmg files. Somehow it got messed up. To fix this, change it to os10 disk copy(default). Should be in the main os10 Applications>Utilities folder if not shown as another option. Then click the change all button and your dmg files should open and in os10.