Checksumming CD or DVD media Raw

**Note on the MD5 command**: On OS X, it's `md5`. On RHEL it's `md5sum`.

### Doing it properly

* For this demo, I'll attempt to burn a CentOS 5.5 DVD
* The ISO name is `centos-5.5.i386.iso`
* It was burned to a DVD drive with device name `/dev/sdc`

Checksumming an ISO image is easy:

    [user@localhost ~]# md5sum centos-5.5.i386.iso  
    75c92246479df172de41b14c9b966344

When you burn this to a DVD, you can verify that everything's good by
first getting the size of the ISO in bytes:

    [user@localhost ~]#  ls -l centos-5.5.i386.iso  
    -rw-r--r-- 1 user user 4185118720 May 20 15:40 centos-5.5.i386.iso

Now divide that by 2048 (4185118720 √∑ 2048 = 2043515). You can now
verify the DVD the ISO was burned to using:

    [user@localhost ~]# dd if=/dev/sdc bs=2048 count=2043515 | md5sum  
    75c92246479df172de41b14c9b966344

This verifies a successful burn. Merely using `dd if=/dev/sdc | md5sum`
will *always* give you an incorrect checksum!