[plug] CD read errors

[Cameron Patrick]

garry wrote:

> If you do:
> 
> md5sum /dev/cdrom
> 
> and compare it to
> 
> md5sum filename.iso
> 
> If they are the same, you are sweet!

BTW if they're different, you don't necessarily have a coaster.  I've
found that some CDROM drives (e.g. the burner in my desktop at home)
don't always read the image back in exactly for some reason (putting
extra padding on the end or some such).  If you mount the CD and the
ISO somewhere and run something like `diff -urN /mnt/cdrom /mnt/image`
you can at least be sure that all files are present and correct, even
if the md5sums might be different.

Also, if you find that your image _is_ missing a bit, using the
command-line tool `readcd` with the option to read one sector at a
time will often (slowly!) extract data off discs which are otherwise
hard to read, or which dd conks out on at the end.

On a side note, dd_rescue is also useful for extracting what data you
can from discs (hard drives or CDROMs) with bad sectors - it reads as
fast as it can until it hits an error, then slows down and retries a
single sector at a time.  After a few failures it skips bad sectors
and moves on the the rest of the drive.  Unlike plain dd, it also has
a progress indicator.  How to get a progress indicator on dd seems to
be a moderately FAQ around here so some people might want to play with
it just for that :-)

Cameron.