Trying to understand input/output error seen while reading newly burned dvd+r

[D Wyatt]

On 7/20/2010 9:31 AM, Tim wrote:
> On Mon, 2010-07-19 at 23:29 -0700, JD wrote:
>> What I also do to reduce the likelihood of a bad burn, is I reduce
>> the burn speed to 2x. Burning at 52X (cdr) or 22X (dvdr) will have
>> a higher likelihood of a bad burn. Some will say this is bunk. I say
>> I have proven it in practice.
> 
> Seconded!  A slower burn is more reliable.  Though I've found medium
> speeds good enough, I don't have the patience for the slowest speeds.
> And some drives/discs exclude them from the supported burning speed
> list, anyway.
> 

My thanks for your replies, however as I thought I indicated,
my problem does not seem to be in the burn process but in the
ISO that I created from a directory on my HD.  The ISO was not
downloaded.

I had seen similar problems before so I had started loop mounting
my ISO's after creating them and doing a diff -r between the
mounted ISO and the origin directory.  If there was an error, I
would recreate the ISO and test it again before burning to DVD+R.

This time, the diff between the mounted ISO and the origin dir
passed w/o error, so I burned it and then did a diff between the
newly burned DVD and the origin dir.  That diff gave an error, so
I burned a second DVD from that ISO, ran diff, and got exactly the
same error on this DVD as the first DVD I'd burned.

I remounted the ISO (mount -o loop ...) and again ran diff -r
against the origin dir to double check and again saw no error.
This is what puzzles me.

Nonetheless, I recreated the ISO from the origin dir, mounted and
tested it, found no error and burned a DVD from the new ISO.  This
time a diff -r between the new DVD and the origin dir passed.

The puzzlement comes from the apparent fact that the first ISO was
flawed in some way that a diff (on the loop mounted ISO) didn't pick
up on.  Still, it yielded exactly the same error in two burns.

Regards,
D Wyatt