A cyclic redundancy check, often shortened to CRC, is a mathematical function used by computers to ensure the integrity of data as it is being transferred from one location to another. If there is a problem with the transfer and the data does not match on both sides, then a cyclic redundancy check error will occur. This can happen in Windows while reading a disc or in any number of programs, including DVD Shrink. Possible causes include a scratched or damaged disc, a faulty optical drive, or the result of trying to back up a copy-protected disc.
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Rule out the optical drive (the DVD player on the computer) as the cause of the error by attempting to read other discs with the drive. If you're successful, the drive can be ruled out as the cause of the CRC error, leaving the disc as the culprit. The disc could be scratched or damaged, or it could've had copy protection against duplication written into it when it was burned. If the optical drive fails to read other discs, then there is likely some problem with the drive itself.
Rule out the surface quality of the disc to be backed up. Even if it looks fine, check to make sure it plays or reads normally outside of DVD Shrink. The DVD could have been incorrectly burned. This happens often with cheaper CD and DVD media; it is rather rare with commercial DVDs. If you can rule out the surface quality of the DVD as a cause for the error, go to step 4. If not, see step 3 for information on attempting to clean the disc.
Clean the faulty disc. It is possible for any amount of scratching, grime or residue to cause read errors. Use a disc cleaning kit, or use water or alcohol and a piece of cotton shirt. Make sure that you wipe against the tracks and not with them; wiping with the tracks can cause further damage.
Having determined that the cause of the error lies in neither the drive nor the disc itself, there remains only one possibility: the cause of the error is the disc's encryption, or copy protection, of one type or another (several different types exist). Going any further in attempting to "crack" the encryption would likely violate copyright laws. In this case, if the DVD you want to back up with DVD Shrink is in good condition and still gives a CRC error, it means that the disc was designed to give the error upon copying, and the manufacturer doesn't want you to be able to back up the disc.