My german is very bad so i am going to write in english. Sorry.
I have a dreambox 7020 as i read here a lot of people say that there are no problems with the cf-hdd combination. But there is a problem and there is a solution. My hdd was already in it as master. So when i wanted to put a cf in it and bring an image on it it wouldn't work. So i set the hdd as slave then the cf was recognized but the hdd not so i installed the image on the cf and then i put the hdd back as master. And everything worked.