Henni,
Format the card on you PC. If that fails to work format the
card on the keyboard. I have the MicroArranger and have used cards from 512 MB up to 32 gigs and they all have worked. I did not have to format them to get them to work. Some came from my Canon camera and they worked too. All I did was copy files from my computer to the card and insert the card in to the keyboard.

Keep us updated.

Deane