Here is a workaround. Drag any driver onto the form, rebuild, open the code tab or any other tab, click back to the form design tab, all the missing drivers are now in the toolbox. Now delete the test driver, drag the correct driver, all the drivers are still in the toolbox when visual studio is restarted. However, if the form contains no driver, then all the missing drivers are missing again when visual studio is restarted. Weird.