Problem with burning disk to disk.

Hi All,
I recently fitted a NEC 16x Dual DVD Burner to a friends computer.
He can burn to hard drive but cannot burn from his DVD Reader disk to disk to his new burner. (This works perfectly otherwise.)
When I insert a DVD 16X Disk into his NEC Burner it states that it has insufficient space. (Only 840 Mb.) Obviously it appears that it is mistaking the dvd for a cd.
I use these particular dvd's regularly withiout any problems.
I am using Nero unloaded with all the plug-ins so can any kind person enlighten me as to what I'm doing wrong?
I burn a lot on my own PC without problems.

many thanks in advance.

Billo