| Recovering media images |  |
| Advanced settings |
The first attempt of reading the defective medium will usually provide enough data for the error correction. If it did not, try the following:
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Estimating the chance of recovery Examine the output of the reading process. Under the "Sectors processed" section you will find the actual percentage of readable sectors and how many percent will be needed for a full recovery. Using the difference between the two values (85.6% - 81.3% = 4.3% in the example) you can estimate the likelyhood of being able to collect enough sectors for a successful recovery:
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| < 5% | Chances are good that you will get enough data using more reading attempts. |
| 5%-10% | If you have several drives with different reading characteristics you may get the required data by being persistent and patient. |
| 10%-20% | You are in trouble. If the missing sectors do not drop significantly below 10% during the next 2-3 reading attempts the medium is probably unrecoverable. |
| > 20% | Too much data loss; you can write this medium off as unrecoverable. To prevent this from happening again, use error correction data with higher redundancies and shorten the intervals for defect scanning. |
Try the following settings one by one in further read attempts. Please perform a complete reading pass for each setting so that you learn how it affects the outcome (sometimes the results also differ depending on the drive used for reading). When you have gone through the list you may combine them into more powerful configurations.
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Perform another reading attempt Do not alter any values except for setting a smaller value for terminating the reading process. Recommended values are: 32 for BD, 16 for DVD and 0 for CD (use the slider marked green). Perform another reading attempt using this setting. You can repeatedly read the medium as long as any pass provides a significant number of new sectors. Hint: Let the drive cool down between the reading passes. Eject and load the medium before each pass; sometimes the medium comes to rest in a better position and the number of readable sectors improves.
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Complete the image using different drives
Perform additional reading attempts using different drives. Transfer the image to other computers to see if their drives can contribute more readable sectors.
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Increase the number of reading attempts For all media types (CD, DVD, BD): Set the number of reading attempts per sector to a minimum of 5 and a maximum of 9 (green markings). Only for CD media: Some drives are capable of partially reading defective sectors on CD media. Activate the "Raw sector caching" option and specify a directory where fragments of defective sectors should be stored (yellow markings). If enough fragments of a defective sector have been collected it may be possible to fully reconstruct it from that information. |
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Examining results of multiple reading attempts (CD, DVD, BD): Not all drives show an improvement after increasing the number of reading attempts. Watch for messages of the form "Sector ..., try x: success" (highlighted in yellow). These indicate that the drive could read a sector after several reading attempts. If you never see such messages, increasing the number of reading attempts does not pay off for the respective drive. |
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Examining partial reading of defective CD sectors:
When the whole medium has been processed, look into the directory you entered
above (/var/tmp/raw in the example). If no raw files have been created
the drive may not support the required reading mode.
However if you have several drives which do create raw files, then let them all
work in the same raw file directory. Collecting raw sector fragments from
different drives hightens the chance of reconstructing the defective sectors.
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Use a different raw reading mode for CD media: Using the preset "20h" raw reading mode might not work on some drives. Perform another reading attempt using raw reading mode "21h" (see the screenshot). Check again whether some raw files have been created. |
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