I can think of a number of things that could be causing this. The first obvious one is that the media is write-protected. Some external hard drives have write protection switches

More likely, it's a problem with permissions, or just not being mounted read-write. Try adding the flag "-orw" to mount, to enable the Read/Write Option. The other possibility is that it's not set for the proper user / group id, so you don't have permission. To set those, specify -ouid= and -ogid=. Unfortunately, I don't have my N800 on me at the moment, so I'm not sure what ID "user" runs as.