DVD er Help. When I want to play a DVD in a LG RH7823W DVD recorder I get unknown disc displayedHi

Question

When I want to play a DVD in a LG RH7823W DVD recorder I get unknown disc displayedHi I have a LG RH7823W stand alone dvd HD recorder 160gb which won't play NTSC format discs. The error that is displayed is" Unknown disk please check disk again".
As this is supposed to be a multi-format player do you have any suggestions as to why this is.Here is a link to specs page for this model if it helps. http://au.lge.com/products/model/detail/video_dvdhddrecorder__rh7823w.jhtml#

Thanks in advance
Grant