Allright, operational again.
Both drives were on one IDE cable and hadn't had problems for 15 months. So when I disconnected one of the drives and keep the other one connected, that one would show up in windos and work. Also if I did it the other way around.
Just when both were connected, none seemed to work anymore.
So I got a new cable, connected both to it, no luck. Now one drive is connected to one cable, and the other to another and they both work. Very strange...