CD and DVD images
Scott Mendham, PC World
21/07/2004 15:29:24
If you have ever lost a valuable CD or DVD
because of a scratch or general wear and tear,
you’ll know the importance of creating a backup
copy. Depending on the type of content you are
backing up, there is a way to both back up your
discs and gain quicker access to your disc
collection without having to juggle CDs or DVDs
in and out of your drive. The technique is known
as imaging, but more frequently it is called
‘creating an ISO’.
An image is an exact copy of data stored on
media — floppy images have been around for
years, and hard drive images play an important
role in backing up entire operating systems. Few
people know it, but CDs and DVDs can also be imaged.
What’s the point?
Before heading off and creating ISOs, there is a
question that needs to be answered — why make an
image of a CD or DVD when all you need to do is
copy or even zip the contents of the disc? The
short response is that, in many cases, you don’t
need to create an image at all. A CD full of
photographs will hardly benefit from being
turned into an image. However, there are a few
instances when creating an image will save time
and minimise the chances of files being lost or
corrupted. The most common is backing up
software installation or game discs.
If you have ever delved into the contents of an
installation CD, you may have found it was chock
full of files scattered across a hundred buried
directories. In most cases, if you start moving
the files or leaving out folders because you
think they serve no purpose, there is a good
chance the installation will stop working. These
kind of unintentional alterations can’t be made
on a CD-R, but it is surprisingly simple to do
if you copy thousands of files to your hard
drive (or zip them without maintaining the
directory structure).
An ISO ‘locks’ the content, meaning you can’t
easily change the files within it. Unless you
have a specialised ISO editing program, the way
to change an ISO is to burn the image to CD (or
load it to a virtual drive — more on that
later), copy the files back to a hard drive to
make changes, and finally create a new ISO. This
is hardly a process that will happen by accident.
Once you have an ISO, it can be backed up or
sent to other users (very popular with Linux
distributions). Depending on their sizes, you
can back up multiple ISOs onto a single CD or DVD.
There is also the issue of file management. An
ISO is a single file. If you have copied 20 CDs
onto your drive and each contains over 1000
files, this adds over 20,000 files to your hard
drive. With the increased load of files, your
searches, system-wide antivirus scans and
defragmentation will all be slower compared to
having just 20 ISO files.
Virtual drives
A virtual drive acts like a CD drive, except
that files are loaded from your hard drive.
Specifically, they load an ISO image and let you
access the contents just as if you had inserted
a CD. This speeds up access to the files because
hard drives are much faster than CD drives, and
you don’t have to swap discs and wait for the CD
to spin up/spin down. Click here to see a screen
shot.
There is a chance you already have a virtual
drive program: Nero includes a frequently
overlooked virtual drive called Nero ImageDrive.
A free alternative is IsoBuster (see the cover
CD for a free copy). This utility works a little
differently — it loads the image, but doesn’t
assign a drive letter.
What type of CDs or DVDs can’t you image?
It may seem obvious, but the whole idea of copy
protection for CDs and DVDs is to stop people
copying them. If the CD or DVD has copy
protection, it is unlikely that the ISO will
function correctly. When the ISO is opened, the
program usually will ask for the original CD to
be inserted into the drive or will give an
obscure error message.
Unfortunately, it is not always obvious that a
disc has copy protection. The two most
frequently protected file types are commercial
DVD movies and game discs. Both systems can be
defeated and many Internet sites are dedicated
to these topics. The most famous is DeCSS, the
tiny program that breaks the encryption
techniques designed to stop duplication of DVDs
(for more information, search Google for
‘DeCSS’). Next month we’ll look at ways to
create ISOs and convert between the various
formats. In the meantime, have a look at the
free versions of IsoBuster and Burn4Free on the
cover CD.
THE STANDARD THAT IS NOT STANDARD
As noted in the introduction, many people refer
to CD and DVD images as ‘ISOs’. In a wider
context, anything associated with an ISO
normally means it conforms to a well-defined
standard that everyone follows. Unfortunately
for CDs and DVDs, this term has been
bastardised, with the result that not all ISOs
are the same. ‘ISO’ images come in wide
varieties such as ISO, BIN/CUE, NRG and CIF.
This can create headaches because of
compatibility issues between different programs.
For example, Nero 6 can burn NRG, ISO or BIN/CUE
files, but Easy CD Creator 6 is limited to CIF
and ISO. Even having the ‘ISO’ extension won’t
clearly tell you the type of file structure the
image contains.
