The NEC 3550 does not burn good DVD+R images for XBOX, you must use DVD-R if you want DVD's burned on the NEC 3550 (1.06 firmware) to be readable on all XBOX drives. This problem also affects the NEC 3500, but apparently not the NEC 3540.
I tore my hair out on this one. Bitsetting doesn't make any difference, nor does using really good media. I have an ISO image which, even when burned on Taiyo Yuden DVD+R media (the quality scans look GREAT), has some files which are unreadable on an XBOX with a Thomson drive. The DVD IS readable on an XBOX with a Philips drive.
At first I thought it was the fault of the Thomson drive, but I swapped it for another Thomson drive and got the exact same failures reading the exact same files. The same image burned on DVD+R by a Benq 1650 or a Sony DRU500A reads just fine, even burned on lousy TDK (CMC) media. The NEC 3550 is definitely producing some kind of artifact that the Thomson drive is sensitive to, but which doesn't show up on a disc scan.
It doesn't matter what software is used to burn it (Nero or Alcohol), and I have moved the drive to another computer and gotten the same results.
Interestingly, I also have an NEC 3540 (1.04 firmware) which does not have this problem. But the NEC 3550 (and also the 3500), writing the same image, will create DVD's which fail every time, often reading the same files.
My personal view is that the Thomson drive XBOX is pointing out a problem with DVD+R's created by NEC drives, a problem which does not affect DVD's created on other drives. I will not write any more DVD+R's on NEC drives, I will either use DVD-R or use a different drive.
Bottom line: If you own an NEC drive, use DVD-R for writing DVD's for XBOX. If you use DVD+R they won't be reliable on XBOX's with Thomson drives.