Help needed

Hi guys. I’m new to the forum as i’m sure will fast become apparent.
I have two NEC DVD drives in my PC. One is a ND-3500AG and the other is a ND-4550A.
Both have always perdormed well and i’ve never had any trouble from them…until today.
Yesterday i bought War of the Worlds on DVD. I always back up my DVD’s using the NEC drives but for some reason neither will recognise this DVD. The DVD plays as normal on a DVD player and if i put it into the DVD-ROM on my girlfriends laptop is recognises it correctly but when i put it into the NEC’s neither drive can read the disk at all…they don’t even recognise the fact the DVD is in.
Can anyone advise on this please???..it’s never happened before and all the other DVD’s i’ve checked since i noticed this still work fine in both the NEC drives.
Cheers
dazza