tar giving different results for same command, trying to test cdrom device integrity

[Walter Tautz <wtautz@math.uwaterloo.ca>]

I am curious if anyone can clear up the following phenomena
for gnu tar on testing:

# tar --version
tar (GNU tar) 1.13.25
Copyright (C) 2001 Free Software Foundation, Inc.
This program comes with NO WARRANTY, to the extent permitted by law.
You may redistribute it under the terms of the GNU General Public License;
see the file named COPYING for details.
Written by John Gilmore and Jay Fenlason.


basically I have  cdrom mounted under a directory 

and i ran 


tar zcvf win2k.tgz /mnt

and

tar zcvf win2k-2.tgz /mnt



Perhaps I should be using different options to get the
same files:


# cmp -version
cmp - GNU diffutils version 2.7


cmp -l on the files yields:

# cmp -l *
     5  35 362
	 6   4   2


I also did a dd on the raw device and that at least gave no differences.

I am trying to determine whether the cdrom drive is faulty. Does anyone know
of good ways to test such a thing.

-walter