Apparently there is code in firmware that make disk world writable on reboot when it finds root folder to have limited accessrigts. See this thread http://forum.dsmg600.info/viewtopic.php?id=6227.
In short, somehow limit fileaccess on rootfolder possibly by using ftp, then reboot the box.
If this doesnt work, then I think the least risk solution is to install fonz funplug and fix the accessrights using telnet.