pags:
Not disputing what you have posted above will work, but the command given, without the count entry will work just as well when copying existing CD/DVD disks. Without the count entry dd will simply copy what it finds on the disk up to and including the last block, then stop.
The block count is primarily a limiting argument, ie if I want to copy the MBR of a drive, I would use;
dd if=/dev/sda of=/path/to/filename bs=512 count=1
... which would limit the number of blocks copied to the single first block of the first sector of the hard drive. If I wanted to add the reserve area that includes the embedded grub stage1_5 files I would do that with a count=63.
With a CD/DVD being copied, one wants the whole thing copied from beginning to end, so dd doesn't need a limiting count number. One can verify this by making two iso images of the same disk using dd, one with the count entry and one without, then do an md5sum check of each image, as well as the original disk. If the count entry is correct for the first image, both images will have the same md5sum number as that of the original disk.
The problem comes in if the count entry is too small, due to a typo when entering the command, in which case the image would get truncated. At best, the count is unneeded, hurts nothing if correct, or too large, but can introduce error if too small.
