Hi,
Thanks for the replies.
I had a little experiment that may help you help me.
I measured SCK of SD car to gnd: 0V
MOSI = 4.4V
MISO = 5V
CS = 5V
Added later: I guess this already shows the error on interfacing.
Do you think this http://www.sparkfun.com/tutorials/65 can already correct the error?