"It should be light and thing for easier portability."

should say instead 'light and thin'

Are you confident the discs will be readable after a few years?  I have DVD-R and CD-R discs burned a few years ago which can't be read by anything.  The media is from reputable manufacturers, too.  :-(

I am well aware of this issue with optical discs (although it hasn't happen to me).

I intend to make at least 2 copies of everything and reburn the discs every few years. Plus I will have stuff on my external 2TB HD.

I have a new networked HD as the old one has failed completely recently being only few months old and I lost all the data. I have decided I cannot trust the HD alone and need additional back ups. 4 bay NAS is rather expensive and is it is all still in the single device it still has some risks. I think a single HD + 2 optical disc copies is the most cost effective and safe solution. It is actually rather efficent now that 25gb storage per disc is readily available with costs of around 1fr per such a disc, and the external optical drive will have other uses as well, such as providing my mac air with an optical drive it doesnt have.