At first, thank you very much for your help! I went a pragmatic way and saved the data on my PC, formated the card and retransfered the data.

The output of df - h says - now - that the card has a size of 3.7G.

Still, if there is a general way to handle such issues, I am still interested how to identify and to remove obsolete data.

Again, thanks a lot!
icke