Are All Mini Discs Recordable?

I have a Sony mini disc player MZ-R501 and now have some mini discs to use with it (thanks again, Entropy Agent!). The player is a 'Recording MD Walkman.

The question is - are all mini discs recordable? In other words if I want to record my own music on these discs do I just record 'over' the music already on these discs?

Thanks

skollie