Hi Christian,

It sounds like your SD1 is messed up with an (incomplete ?) Operating system.
You could try to download OS 3 at first from the www.ketron.it website. (software)
Follow the instructions given :
Insert a floppydisk with the unzipped files into the drive.
While switching on the SD1, keep F6 pressed and your 'upgrading' will start automatically.

Hope this helps !

Roel