Why do I get CRC errors when I unzip compressed archives? What is a CRC error?

These occur relatively infrequently, but most people have experienced such problems at one time or another. CRC is an acronym for cyclic redundancy check, a mathematical method used to calculate the validity of the data in a given archive.

CRC errors can transpire when files are resumed too many times during the download process and as a result lose a few bytes along the way. Consequentially they become corrupt and are ultimately useless. Some of the more advanced download managers have a kind of 'roll back' function to counter this problem, which re-copies the last few bytes of a file each time it is resumed. It's also possible to choose to what extent you would like to backtrack. This is useful because the amount of data which can become corrupt upon resuming a transfer is directly proportional to the speed of your connection.

Compressed archives, or any file type for that matter, can also become corrupt if they are downloaded too fast. If your computer is unable to write data to the hard drive as quickly as it is being downloaded, some of the data can be lost. What you must ensure is that your computer is sufficiently powerful to keep up with the demands of your modem. If not, you will have to employ your download manager to impose a speed limit on your transfers.

Several different methods can be used to repair damaged archives with varying degrees of success. To access Winace's archive repair function, browse along to 'archive' then 'repair archive' within the menus at the top of the interface. Similarly, Winrar's equivalent tool can be located under the 'commands' > 'repair archive' option. Don't be surprised, however, if they both fail dismally.

The developers of Winrar, in realising how useless these tools are, have now created a new system used to repair damaged archives and have made it available in Winrar versions 3.0 and above. It uses 'rev' files (recovery volume) to maintain a record of how to repair a given archive if it should become corrupt.

These work in much the same way as 'par' files (the topic of another blog entry) and in addition to providing the means to repair damaged archives, also allow you to rebuild missing segments of archives. Rev files can be created along with the archives they are designed to rebuild providing the relevant box is ticked beforehand. If a rev file is present when extracting the archives it will automatically be used by Winrar to repair and rebuild the archives on the fly.

Nevertheless, a less sophisticated yet dependable way of ensuring that your archives are error free, is to download them again and re-test them for CRC problems. If you experience recurring errors it would be wise to download the same files from a different location.