Slimtype DVD DS8A1P does NOT support Dual Layer Burning
I have the exact same DVD burner drive in my Acer Aspire 6920 notebook. I've been having the same problem, of not being able to burn Dual Layer discs. Even though the drive is advertised, on a product sticker stuck directly on the notebook, as a "DVD-Super Multi DL" (with the DL obviousely standing for "Dual Layer"), it seems that this particular model of DVD burner does NOT support Dual Layer burning.
I have been able to use a friend's external USB Mad Dog DL burner to burn my dual layer discs, proving the software works fine. Fortunately the Slimtype DVD drive will READ Dual Layer discs. As far as discs burned directly on the notebook, it DOES burn traditional Single Layer (4.7gb) discs just fine.
I need to send in my notebook back to Acer soon for repairs, and luckily, I've invested in the 3-year extended warranty. The keyboard needs to be fixed, among other things. The next time I talk to Acer, regarding sending in the notebook, I am going to see if they will install a different DVD drive that does support burning DL discs. I don't know, but I might need to pay a little extra money for them to do this. I hope not, but even so it will be a worthy investment.
Good Luck,
NateDog