Buy a can of compressed air and blow gently into the drive. Likely a dirty LED reader. Probably coincidental with the installation of High Sierra and not the fault of the installation.
Turns out it's the DVD and not the optical drive. I've now tried several other DVD and they mount and are readable. It' just the movie DVD which was sent by Netflix that is not mounting. Just put the Netflix DVD in my DVD player on my TV and it's plays fine so nothing is defective in the DVD. Very odd that my Mac's optical drive can't read it then.