>>> of an almost full 4.7 GB DVD)  IIRC it didn't have
>>> any read errors, so just dd-ing it to /dev/null
>>> wouldn't have caught the problem.
>>
>> ins't your DVD drive broken? at least in theory correction code on DVD
>> discs should at least always detect an error.
>
> Hmmm, if I had gotten a read error the tmp file would have been at
> least a block short.  So there must have not been a read error.
>

i mean DVD broken because it doesn't detect wrong data!

other explanation - transmission error occured while writing through IDE 
cable. or maybe RAM is broken etc..

> I haven't studied whatever error detection code they use on
> DVDs, but in general they can't detect all possible errors.
>
> I've seen more than one error detection and correction system
> fail to deliver correct data.  :-(
>