data CRC error while ripping

[nirmala]

I have a Buslink(AOPEN) 16x dual layer dual format burner(external USB2). I have Nero 6 OEM, dvd decrypter and DVDShrink. I want to backup a few of my Tamil(south Indian) movie DVDs . Infotool says that the discs support all regions(they are made primarily in Canada/UK/Malaysia) and are mostly 2 layers 8.7G disc. When I start encoding in Nero 6 recode/dvdshrink, I get data CRC error (anywhere in the 50-70% thru the analyze phase). Most of the time there is a VOB that hits this error(the vob # seems to vary for different disks). I don't see visible scratches on the disk and PowerDVD seems to play them ok.
With DVDdecrypter, I hit read errors on lot of sectors in the problematic vob(i have set it to ignore read errors after certain # of retries). The error is Focus servo error. I have tried ifoedit or isobuster on those DVDs and they all seem to fail once they hit these bad sectors.
Any ideas on how to back them up(they are not very old dvds)?

[nirmala]

Another thing I forgot to mention was that I tried backing up using RecordNow on a different burner(Sony DRU 500 or so) in my friend's machine and it seem to work(atleast on a few DVDs).