How does a cd rw become read only?

[ricky1me]

I am relatively new at using cd's (I'm from the old days and was in the habit of using floppys!)
Anyway I was using a cd rw for backups and all of a sudden the Windows xp cd burner program told me the cd was now read only yet had lots of space left on it.
Any tips?
Thanks

[merlin_2]

if you finalise the disk it becomes a read only disk...do you..^^^^close the disk...