Remarks on read errors

When the capacity of the built-in error correction is finally exhausted, read errors will start to appear on the medium. These will be reported by the "Scan"-operation of dvdisaster. Depending on the time of first occurrence, two types of read errors are of particular interest:

Read errors appearing right after writing the medium. This is a sign of:

• media from a faulty production run, or
• media which are not compatible with the DVD writer.

Please withstand the temptation of trying to preserve the faulty media by means of an error correction file - this will most likely end with data loss.

Read errors after a few months/years. The built-in error correction of the medium will be increasingly loaded during its life time until it finally fails and read errors show up. This happens for mechanical reasons (scratches, warping of the plastic material) as well as for chemical causes (decaying dye and/or reflective layer).

These effects typically occur while the medium is stored away for a few months, and it may not be possible to read in all sectors afterwards.

Therefore it is crucial to create the error correction file (ecc file) in time. The ecc file contains information for recalculating the contents of missing sectors (within certain limits). Therefore with the help of the ecc file dvdisaster can recover images even if not all sectors could actually be read by the drive.

Since the error correction can reconstruct missing sectors up to a certain number, it is not necessary to squeeze out a defective medium for every readable sector. The adaptive reading strategy checks during reading whether enough data for error correction has been collected. As soon as this is the case, reading stops and still unread sectors will be recovered using the ecc file.

Some hints for effectively reading damaged media

The outcome from reading damaged media depends on several factors:

• Not all drives are built the same.
  Different drives have different reading capabilities. Take advantage of dvdisaster's function for completing an image with several reading passes and use different drives for each pass. Transfer the image file between computers using a network or rewritable media in order to use drives installed in different machines.

• Eject and insert the medium again.
  Sometimes it makes a difference to eject the medium, turn it about a quarter, and then load it again for another reading pass.

• Some drives read better while being cold.
  Turn off the computer over night and perform another reading attempt in the next morning.

  But note: "Cold" refers to normal living room conditions - putting hardware or media into the fridge can be very unhealthy for them.