So, just to be certain, after changing ro to rw, your udevil.conf looks like this?
By default permissions are set to read-only, so copying or attempting drag and drop to external storage will fail. To copy files to a USB drive, the default permission has to be changed first, from ro (read-only) to rw(read-write) in /etc/udevil/udevil.conf. Example (line 219-221):
default_options_vfat = rw, iocharset=utf8, noatime, uid=$UID, gid=$GID
default_options_exfat = rw, nonempty, uid=$UID, gid=$GID
default_options_hfsplus = rw, uid=$UID, gid=$GID
