Try to unmount & unplug, then re-plug the device, while looking at the log file (sudo tail -f /var/log/syslog).
Chances are that it gets mounted read-only because of potential errors on the file system, but this is all speculation. Only the log file will reveal the reason.
Side note: you
did unmount ("Auswerfen") the device before unplugging it, right? Unplugging USB devices without unmounting them first is one of the most promising and easy ways to screw them up
