Ok, I formated my 32GB class 10 card with ext2 (I did this on my PC). After putting the card into my N900, I started to copy (tar) the debian files to the card. But this process aborted quite soon (I/O error).

Doing the same steps with my 16GB class 6 card is no problem.

AndrewX192, the questions now really would be if this can be fixed with a kernel patch, or if this really is a hardware compatibility problem.