I'd wish all things in life could that easy: In a last attempt I unscrewed my MacBook for I don't know how many times. I disconnected the combodrive cable and plugged it back in again, using a little more force than usual. That was the cause of the problem: The thing was not sitting tightly enough in its socket. Now no more crashes, DVD can be watched and Audio CDs listened to. Thank you.
Santa Rosa's HD is the 100% copy of the HD in my 1.83 Ghz MacBook, password, admin rights and all. Both HDs are, so to speak, identical twins. I also checked all the connections, unplugged and pushed them into their sockets again, just to make sure, they fit in tightly. You must hear them snap. I found no difference between the 2.5" SATA Seagate HD in Santa Rosa and the 2.5" SATA Toshiba HD in the other device. The plugs - I am not so familiar with technical terms in English - look all the same to me. What puzzles me that my 1.83 GHz MacBook recognized with zero problems the HD from Santa Rosa, but not the other way around. May it be as it is. the 2.2 GHz computer works fine. And the other MacBook will serve as substitute in case Santa Rosa crashes - or gets drowned in German beer... Thank you.