Data Safekeeping in the CD-RW Age

Paul Crowley, 1999

One reoccurring question from our customers and others using CD-R and CD-RW technologies is “How secure is my data?”  The answer to this question is not obvious, certainly not as obvious as current software and hardware manufacturers would have you believe.  There are specific things you can do to minimize your exposure to threats to data security, including selecting the appropriate media and proper techniques.

Threats to Data Security

There are a number of simple and obvious threats to data security for the average user.  These can be categorized as environmental, accidental, and technical.

Environmental threats are fairly simple and can easily be protected against by taking ordinary precautions when handling CD-R and CD-RW media.  Both media types are insensitive to magnetic fields, but they are sensitive to both UV light and large temperature variations.

Accidental threats to data security are less obvious and can result in complete loss of data on a piece of media.  To guard against accidental data loss, you have to be aware that there are two aspects which can harm your media: damage to the top, or reflector, and damage to the bottom, or data side.  What is important to understand is that the reflector is far more sensitive to damage and scratches than the data side.  While there is substantial amounts of error correction that is present for handling the errors that are introduced by scratches on the bottom of a disc, there is virtually no ability to recover from damage to the reflector.  This means that a scratch on the bottom of the disc may have no effect on the ability of the disc to be read whereas a scratch that damages the reflector may render the disc completely unreadable.

One of the more common things that can irreparably damage a disc is simply writing on the top surface with a ball-point pen.  Certain types of special pens are available for writing on CD-R and CD-RW media, but in general people have found these to be a waste of money.  While in the beginning of CD-R disc manufacturing the reflector was sensitive to alcohol-based markers, this is generally not the case today.  Many people have found the simple “Sharpie”-type markers work quite well.  If you are concerned about any potential effects to the reflector, write on a label or silk-screened area of the disc.

While it may seem that a CD-R or CD-RW is fragile and can be easily broken, this is not entirely true.  CD discs are made of polycarbonate plastic which is one of the toughest and resistant to cracks and bending.  However, while flexing the disc will not damage the plastic, it may cause the reflector to delaminate.  This is generally fatal to the media.

Technical Threats

The threats most difficult to protect against are those that affect the software and hardware of the computer system.  These may not be well understood by the average user, and they can be the most subtle.

The first issue is hardware failures.  Due to the nature of recording on CD-R and CD-RW discs, there are a variety of hardware failures that can go undetected which prevent the information from being recorded on the disc.  This can be devastating when data which is assumed to be recorded has turned out to not be present on the disc at a later time.  While tape drives and other magnetic media generally implement some type of read-after-write check, this is not done on CD-R and CD-RW recorders because of the physical nature of the recording.  This means it is possible for the software to write to the disc and no actual recording is done.

There are also software threats that put discs at risk.  These can be obvious to the end user as crashes and also more subtle in terms of errors in the recording.  It is important to understand that these risks can be minimized when preparing critical discs.

Some of these risks can be minimized by specific techniques and procedures for recording discs.

Minimizing Your Exposure

The first thing you can do is minimize your exposure to environmental threats.  This means keeping your discs in a place where the temperature is stable and there is no UV light striking them - generally out of the sun is sufficient.  There are no special considerations for the temperature and humidity outside of normal office conditions.  In general, when you are comfortable, your data is safe from environmental concerns.

It is a good idea to keep your discs in some kind of container or sleeve to keep dust and dirt away from it and to reduce the chance of physical damage.  Other than that, discs do not need special handling to protect them with some reasonable cautions against damaging them by accident.

Protecting your data from accidental damage is best done through education - if you understand what will harm discs you can avoid this kind of damage.

Hardware threats to your data can best be minimized by verification of the contents of the disc after recording.  There are various ways of verifying the successful recording of a disc, from simply reading it to performing a byte-for-byte verification of the data.  Most hardware problems that can cause data loss without reporting it via a recording error will result in the disc being recording incompletely or not at all.  This can easily be verified by testing to see if the disc can be read by the operating system it was recorded for.  Reading all of the contents of the disc is a little bit more verification and can check for incompletely recorded sectors due to dirt, fingerprints and other obstructions to the laser light.  Byte-for-byte verification of the disc involves reading all of the disc and comparing it to the source material.  This will uncover software recording problems, but after initially verifying the correctness of the recording software is generally not necessary.

Software threats can be minimized by verifying the recording of a disc when a new release is being used.  Also, it is strongly recommended to be able to remove changes to your software (recording, operating system, etc.) if they prove to cause problems.  After each change it is important to revivify the correctness of your recording environment.

Strategies for Safety

The choice between different alternatives in CD recording has a direct impact on data security.  It is important to understand the relationships between different alternatives and how this affects the security of the data recorded, both when it is recorded and in the future.

There choice of different techniques for recording and the media used is affected by how long the data is to be kept, the consequences of data loss, and how the data is to be used.

While only limited studies of CD-RW longevity have been done, it is generally assumed that CD-R media will retain recorded information longer than CD-RW media.  The information that is available indicates that the difference is only significant after 3 to 5 years.  So, if the data you are recording is important to have in 10 years it is probably better on CD-R rather than CD-RW.  However, if the data has no value after two years it doesn’t matter what media is used.

Another consideration for media choice is the platforms which will be used to read it.  Today, virtually all CD-ROM drives can read CD-RW media, but this has not been true until recently.  Therefore, if you need to access the data from older hardware CD-RW may not be a suitable choice.  Similarly, CD audio players do not generally read CD-RW discs at this time.

Similarly, the choice of recording techniques can be guided by the consequences of data loss.  While it is convenient to use multiple sessions and packet-writing techniques to maximize the utilization of media, it is also important to understand that each time the disc is recorded on there is a non-zero chance of previously recorded data being affected.  In the case of multiple sessions, adding a new session cannot directly corrupt a previously recorded session, but it is possible to lose the ability to easily access previously recorded data.

Packet-writing on either CD-R or CD-RW media has been introduced in the last few years and offers the user the ability to use CD-RW media as a large removable read-write device.  However, it is important to understand the limitations of this usage.  The most significant of this is the potential to corrupt the media when updating it by adding or deleting files.  Even when writing to a CD-R disc, there is the potential for losing access to previously recorded data, even though the actual data cannot be altered.

Based on this, if the data being recorded is critical it is best to record once on a piece of media and then put it into a secure location.  Recording again on the same piece of media presents a small, but non-zero, risk of affecting the previously recorded data.  When the data is critical, this level of security may be appropriate.

Use of multisession recording, using either ISO-9660 or Joliet format, is slightly less secure than simply not reusing the media, but may present an acceptable risk.  There are a number of tools for providing access to previously recorded sessions even when the last session on a disc is not complete and has been corrupted by incomplete recording.  If this is sufficient for your needs, then this will allow using a single piece of media multiple times.

Packet-writing using the UDF file system presents another possible recording technique and when used in the same way as multisession recording, it presents no greater risk of data loss.  It is important to understand that when using CD-R media that it should be closed periodically to minimize the potential for unrecoverable data loss, although this does not completely eliminate the potential for problems.  CD-RW media cannot be “closed” in this manner and there is a significantly greater potential for data loss when media containing previously recorded files is updated.

Another potentially useful aspect of this is that less data is actually transferred to a UDF CD-RW disc when files are added than when a UDF CD-R is updated.  This may be of interest in cases where a disc is being used to record small amounts of data at a time.  This has been observed with a number of different UDF packet-writing programs but is not assured for all implementations.

When using many of the currently available backup programs, it is best for data security purposes if media is not added to, but erased before each use.

The only significant comment at this time about data usage is that if the data is to be used outside of the Windows or Mac environment, such as for Linux or other UNIX systems, it should be recorded in ISO-9660 or Joliet format.  While there are a number of UDF implementations available, the compatibility of these has not been tested over time.  In the future UDF may be an acceptable format for data interchange there are a number of problems today.  These will be resolved in the near future, but if your consideration is data security and the ability to read your data in the future, UDF is not a suitable format for this yet.


For the utmost in data security, the following is suggested:

If this is too extreme for your needs, consider using multisession recording to allow more use from each piece of media.

If you require the features of packet-writing programs, such as drive-letter access, then it is recommended to use each piece of media once without updating or adding files.  If this is not practical, consider using CD-R media and closing it frequently.

While CD-R Diagnostic can recover data after many types of software failures, hardware failures and even some types of accidental media damage, it is not a substitute for taking precautions that may not require it to be used.

Copyright 1999 CD-ROM Productions, All Rights Reserved.
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