I did not put any files on the SD-card. How could I, I have no rights to write on it.
Maybe you misunderstood me. The music is located on an usb hard drive. But some deleted data was moved into trash bin on sd-card, where there is now 0 bytes free.
There is another partition for the OS as you guessed, there is space left. Because IdPi shows the "there is no disk space"-message I thought the problem lies in the full partition.
I got the SD-card sent from JRiver and put it into my Raspberry Pi.
I will upload a screen soon.