Thanks for your reply. I changed the settings for
GSATA 6_7/IDE Ctrl Mode (Marvell 9128 Chip), and
GSATA 8_9/IDE Ctrl Mode (GIGABYTE SATA2 Chip)
from AHCI to IDE.
Tried the drive again on both interfaces - still got several miscompares when verifying Verbatim DVD+RW burns.
To elliminate my unfamiliarity with AHCI and optical drives , I then tried the drive on a much older PC running W2k based on an ASRock P4i65G motherboard that had two unused Southbridge: Intel ICH5 SATA 1.5 Gb/s connectors - no AHCI BIOS options here!
I tried another few burns with brand new Verbatim DVD+RWs, each time got several miscompares out of 4600 files.
Then I hooked up a Samsung USB SE-S084B burner to the old PC and burnt four Verbatim DVD+RWs without any verify error! I even double checked using md5summer!
This has all been very time consuming - but I'm now convinced the drive is faulty. It's brand new bought last week and shows Manufactured Date June 2010. I have all the DVD+RW disks still and burn logs if anyone is interested.
The media I was using are Verbatim DVD+RW x4 (MKM-A02) from two different batches old blue 10pks and newer yellow 25pks - alll Made in Taiwan.
Guess it's RMA time for this particular cheap ($15 / GBP11) and nasty drive.