You can only back up to things connected to USB with your model.(source:http://manuals.playstation.net/document/en/ps3/current/settings/backuputility.html)
Maybe you know someone with an external you can borrow? Otherwise it looks like buying an enclosure would be your best bet. It depends on the content on your drive as well. If you have a lot of movies and music you can delete it assuming it is on your NAS as well and transfer it later? But in that case you are probably running a media server(Or should be!).