I have just noticed & don't know if this has always been the case but my Samsung CDRW/DVD SM-352b will not recognise some of my DVD's. It recognises commercial discs & discs that I have burnt using Nero on 2x speed DVD-RW Verbatim. It will not recognise any 4x discs that I have tried. I have tried Ritek 4x DVD-r and also -RW. Is this normal? The drive is supposed to be multi read all CD,DVD formats and up to 16x DVD-ROM. Any comments or explanations would be appreciated.