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Recovering Quick-Erased Discs, Part 2

October 6, 2011

Previously it was noted that there are two ways to access the data that is present on quick-erased media. The problem isn’t that the data isn’t there, because it is. The problem is that the drives refuse to allow the data to be accessed. There is no special command to force the drive to allow access, so all we can do is trick or coerce the drive into accessing the data.

In the previous article one such technique was described to convince the drive to access the data, although it is not a very good one. This article is dedicated to the description of a much better technique without any risk to the data on the disc.

One important aspect of how CD and DVD drives work is that when a disc is inserted the drive attempts to read the table of contents information from the lead-in area of the disc. If multiple sessions are present, the drive will follow the chain of sessions reading each of the lead-in areas in turn. Some drives respond poorly to read errors at this time and will quickly reject a disc from which the table of contents information cannot be read from easily. Other drives will try harder and retry the operation multiple times. In either case, if the table of contents information from the first session cannot be read the result is that the disc is considered inaccessible and no further access is attempted.

If the table of contents information can be read, this information is held in memory in the drive and the table of contents information is not accessed from the disc again. This fact is key for the technique that is described in this article. Once the table of contents information is read from a disc, it is never read again and any reference to it is made from the memory of the drive.

So if the only area of a quick-erased disc that is affected is the lead-in, then if we have the drive read the lead-in from a different disc and then substitute the quick-erased disc the drive will then allow access to the entire content of the quick-erased disc because it will be referencing the table of contents from the lead-in of the different disc.\

There are also some other aspects of a disc which are determined when reading the lead-in area. The most important of these is the adjustment to the signal processing that is done based on the reflectivity and contrast ratio of the disc. Because drives can perform significant adjustments, it is possible for the drive to accept quite a wide range of reflectivity and contrast ratios which mean that the drive can read from different types and different dye color discs. As many of these adjustments are done once at the time the lead-in information is being read, it is important that the different disc be as close in color and type to the quick-erased disc as possible. Thus if you have a DVD-RW that is rated for 4x writing, a 4x DVD-RW should be used as the substitute disc.

Preparation of the substitute disc is quite simple – it should be formatted completely and/or filled with data. For DVD+RW there is no separate format operation, so the disc must simply be filled. The software used for this is not important and it is not required that the same software be used as was used to record the information on the quick-erased disc. In nearly all cases some type of drag-and-drop writing software utilizing packet writing (on CDs) or incremental recording (on DVDs) is used. However, it is possible to utilize a rewritable disc in a manner just like a write-once disc with mastering software. This is not commonly done but would require that the substitute disc be constructed the same way.

Now we get to the difficult part. When switching discs it is important to not inform the drive of the substitution. How can this be done? Well, there is one way that is available and that is to disassemble the drive. Once the cover is removed from the drive it is possible to switch discs without opening the tray or otherwise informing the drive of the substitution. This technique is described in Appendix A of the book CD and DVD Forensics which can be found at Amazon, other book sellers, or InfinaDyne. There are complete instructions to guide in the disassembly of the drive and description of the swapping procedure to be used.

The one thing that should be mentioned about opening the drive up in this manner is that the laser used to read from the disc is pointed straight up and some laser energy does penetrate the disc. This means that no one should look straight down at the spinning disc while the drive is in operation. Scattering of the laser energy to the sides is minimal to non-existent so only the vertical aspect of this is important. With care, this technique can be used with safety. Certainly it is not recommended to have such a drive in operation with children nearby.

Regardless of the possibility of using a disassembled drive safely, in some environments it will simply not be acceptable to do this. In the last year InfinaDyne has created the “Rescue Drive” to address this. The Rescue Drive is a slightly modified drive which can be either in an external case or internal to a computer. It is completely enclosed and therefore there are no laser safety considerations.

Once the substitution has been accomplished, some software other than Windows (or Mac OS X) is going to be necessary to use to access the disc. Obviously I am going to recommend either CD/DVD Diagnostic or CD/DVD Inspector available through InfinaDyne. You may be able use other products with complete success.

If you have a quick-erased disc that you need to access, please feel free to give us a call at InfinaDyne to ask about what is required. We have gotten this down to a simple procedure with 100% success in every case.

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