Weird DVD-R Problem

[LF28]

Hi Everyone,

I have a really strange problem when it comes to burning on DVD-R media. My system specs are:

CPU: Athlon 2600+ XP
System: Win XP, 1 Gig RAM
Soft: Nero 6.0
Board: ASUS A7N8X Deluxe
DVD burner: NEC ND-2500A (dual DVD+-RW)
DVD-ROM: Lite-On something

DVD-RW is master (Ultra DMA Mode 2)on secondary IDE, DVD-ROM is slave (Ultra DMA Mode 2) on secondary IDE.

So here is my problem:
The burner acceps and burns DVD-R media. So far as Nero is concerned, all burn processes were flawless. But then neither the burner nor the DVD-ROM can read data from those discs although the system recognizes those DVD and can read the directory correctly.
Now, when I use DVD+R discs everything works fine.
I use EMTEC DVD-R and imation DVD-R which both are no cheap noname brands. (the imation DVD-R actually beeing recommended by NEC for the predecessor model 1300A)

The funny thing is, I have tried the LiteOn LDW 811S too, producing the exact same results.
Can it really be that whichever burner I use in my system is doomed never to be able to use DVD-R media?

I am really puzzled and appreciate any suggestions.

Tobi