Error when saving your DWG file on a network - Drawing file is write protected.
AutoCAD uses a special file locking for its drawing files. So if you are using a non-standard file server (Novell, Linux...), or when there are timing problems on your LAN (overloaded network, overloaded server, HW error of your LAN card or a faulty active LAN element - hub, router), you may get false "write-protection" errors when working with drawing files stored on a network server.
Save your drawing locally (the SAVEAS command).
Of course you cannot save to a CD or to a write-protected folder.