Try this: Put the R4 in your DS (without the MicroSD Card) Then turn it on (it will say Loading...) Turn it off again! Then slowly push the MicroSD Card inside the R4 when its still in your DS! (dont push when its inside it) And turn Your DS on and try again! This may seem strange but it worked for me XD PS. did you try another firmware?
Cool...he has tried everything. New firmware won't solve this as his micro SD card works in his friends card.
Kernels will not fix this problem. http://forum.romulation.net/index.php?topic=46582.msg1039956#msg1039956 Read that.
Paper trick: Put a piece of paper agains the R4 and insert it into the DS. Do NOT use too thick paper, but an amount so it presses against the slot.