Information about CRC errors when extracting files

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When extracting files you might notice the CRC errors. In this article, we'll try to provide you with more details about the errors.

About CRC Errors

A CRC error indicates that some data in your Zip file (.zip or .zipx) is damaged. CRC stands for cyclic redundancy check. It is a calculation made from all the data in a file to insure accuracy. When you add a file to a Zip file, WinZip calculates a CRC value for the file and saves the value in the Zip file. When you later extract the file from the Zip file, WinZip calculates the CRC of the extracted file and compares it to the value stored when the file was zipped. If these two CRC values do not match, the file that was extracted does not match the original file, and WinZip will display a CRC Error message.

When the data in a Zip file is damaged, it may not be possible to extract all of the files from the Zip file correctly. Damaged data can affect the entire Zip file, multiple files, or just one file.

Why CRC Errors Occur

There are many possible causes for data damage. Among the most common is a transfer error when downloading a Zip file from the internet. Such an error can introduce invalid data into a Zip file. Some other possible causes include exposure of media to excessive temperatures or magnetic fields, cross-linked disk sectors, and mechanical problems with disk drives.

What to do if a CRC Error Occurs

The best solution to the problem of a damaged Zip file is to try to obtain another copy of the file. For example, use your backup copy of the file or get a new copy from the original source. If you obtained the Zip file by downloading it, then downloading it again will almost always solve the problem.

A "Last Resort" for CRC Errors

If you cannot download a new copy of the Zip file, obtain a replacement disk, or use a backup copy, you may still be able to recover some of your files, and even save portions of the files that are damaged in the Zip file. Here is the recommended procedure.

  1. Start WinZip and open the damaged Zip file
  2. Use the current available method for unzipping files to a particular folder
  3. In the Unzip dialog, select a target folder and click Unzip

WinZip will extract any undamaged files to the target folder. Each time WinZip encounters a damaged file, it will display a CRC Error message. Before clicking OK in the error message, open the target folder and look for the file; some portion of it will probably already have been extracted. You can copy the partially extracted file to another folder and then click OK in the WinZip error dialog. When you click OK, the partially extracted file will be deleted from the target folder, but the copy of the file you made in the other folder will still be there.

Whether or not these partially recovered files will be useful depends on the nature of the file, what kind of damage there is, how much of the file was recovered successfully, and what kind of program(s) use the file. In some cases, a partial file is of no use at all; in some cases, loading a partial file into your application program and re-saving it will completely restore the data. In other (perhaps most) cases, the result will lie somewhere between these two extremes--some of the data will be recovered, and some will be lost.

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