Problems with copying DVD's

[lauz81]

I'm having a few problems. All DVD-R discs that I create with my Sony RDR-GX7 DVD recorder play fine in 2 different players, and I am finalising them all prior to finishing. However, I am unable to copy these discs in my Dell DVD+RW external burner drive (linked to laptop). I haven't had any problems copying other discs with this burner, and have used a couple of different brands of disc (Maxell and Sony) from my recorder. I have also been able to copy other DVD-R discs in this burner (to DVD+R) which were recorded in other DVD recorders, and DVD+RW discs created in my DVD recorder copy fine in the burner... so what am I doing wrong for my DVD-R discs to not work?

Any ideas? Thanks...