CD (Compact Disc) and DVD (Digital Versatile Disc) media are common optical formats for storing music, video, photos and data files. They are a convenient choice for fast data backup when compared with alternative storage formats, such as tape. Since many desktop and notebook systems are now shipped with DVD-RW drives as standard equipment, inexpensive backup of critical files can be accomplished in minutes.

Manufacturers claim that CD and DVD media should, when stored properly in a controlled environment, hold data for 100 years or longer. However, they are extremely susceptible to environmental and handling problems, the most common being fingerprints, smears and scratch damage.

CD & DVD Recordable Media - How They Work

Unlike magnetic media, CD and DVD media use reflected patterns to optically interpret recorded data. CD-R (Compact Disc - Recordable) media allows a single, one-time write operation to be performed by a laser that heats a layer of organic dye. The laser heats the dye to approximately 200� C creating permanent "pits" in the recording layer. Another layer of plastic fills these voids and creates the reflective pattern that is then read by the drive. This type of write process is permanent and the media cannot be reused. CD-R media can be read in most CD and DVD drives.

CD-RW (Compact Disc - Read / Write) media can be written, erased and overwritten many times over. Unlike the dye recording layer found in CD-R disks, the CD-RW recording layer is made of various metal alloys. When heated to approximately 700� C, the targeted areas will change from a crystalline, highly-reflective state to a non-crystalline, less-reflective (or amorphous) state. This non-crystalline pattern is then read by the CD-RW drive as data. To erase data, the laser heats the selected area to approximately 200� C (similar to a CD-R drive) and returns the area to crystalline condition. During an overwrite operation, the laser performs both operations at the same time - writing new data while erasing the older recording.

DVD-R and DVD-RW represent enhancements to both the capacity and the technology used with recordable CD media. DVD-R media is very similar to CD-R in that it also uses an organic dye technology for recording. DVD-RW media utilizes a phase-change technology, similar to CD-RW, that allows many hundreds of write, erase and overwrite operations over the life of the media.

Rescuing Lost Data

When data loss occurs with recorded optical media, there can be many reasons. Many times, errors develop during the write process itself. If a recording session does not close properly, or is aborted before it has chance to close, the data already written to the disk may be inaccessible. Selection of the correct media is important as well. Data should always be written using media that has a speed rating that is appropriate for the drive being used. Using generic (non-brand name) media, or media not rated for the speed you will be recording at, can lead to recording problems during a write session or read problems later on.

Examine the disk for any scratches, smears, warping, or other physical problems. Many read problems with previously recorded media are simply due to fingerprints that distort the ability of the laser to accurately read the disk.

There are commercially available recovery software programs that attempt to recover inaccessible data from damaged or corrupted optical disks. Some are more successful than others, but usually they are used in the same system and using the same optical drive that originally wrote to the disk. This can be problematic in a recovery effort. For example, if the original drive is defective, or has an intermittent read problem, the software may misinterpret the pattern written to the disk. This can result in recovering files with corrupted data, files with zero bytes of data, or simply no files at all.

If the data is critical, a professional data recovery service may be the best solution. A data recovery company experienced in optical technologies will initially examine the media for any handling or physical damage. Tools and techniques are then employed that allow for a thorough analysis of the recorded pattern at the block level of the format. By identifying the best representation of the recorded signal, precise and accurate extraction and retrieval of previously recorded data can be achieved.

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