Hi!
Use the HDToolBox and change the "BootPri" for dh0: to something higher than 20 (i.e. 30). As 20 is the BootPri of df0: ...
HDToolBox will ask and warn you that probably the HD isn´t useable after saving the changes. But if you only change the BootPri there will be no risk.... ;-)
-Micha1701
