Hello Ralf,
i think this is because your usb media is mounted read only.
Depending on the filesystem used, try to modify /etc/udevil/udevil.conf
Find this part
default_options_udf = nosuid, noexec, nodev, noatime, uid=$UID, gid=$GID
default_options_vfat = ro, iocharset=utf8, noatime, uid=$UID, gid=$GID
default_options_exfat = ro, nonempty, uid=$UID, gid=$GID
default_options_hfsplus = ro, uid=$UID, gid=$GID
and change "ro" to "rw"
default_options_exfat = rw, nonempty, uid=$UID, gid=$GID
or
default_options_vfat = rw, iocharset=utf8, noatime, uid=$UID, gid=$GID
You can do this with WinSCP.
Frank