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Information about CRC errors (encountered when extracting files)

Article ID: 41
Last updated: 16 Oct, 2012
Views: 53481

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 in transmission. 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.

CRC Errors on Removable disks

If the damaged Zip file is on a removable disk, the removable disk itself may be physically damaged, or the data on it may have been corrupted. In these cases, the only completely reliable solution is to use another, undamaged copy of the disk or Zip file. There are, however, two other situations in which it may be possible to recover some files from a removable disk.

  1. The removable disk drive that you are using to unzip the Zip file may be malfunctioning. Try using a different disk drive, if possible.
  2. If the removable disk drive you are using to unzip the Zip file is not the same drive that was used to store the Zip file on the removable disk, and you have access to the original drive, try using this drive to unzip. It is possible that the original drive may be able to read the disk; if so, you can extract your files using this disk drive (and have your removable disk drives checked out - at least one of them may be out of alignment). Otherwise, the removable disk itself or the data on it is probably damaged.

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. Click Folder name or Location (Legacy menus/toolbar - click the Extract button)
  3. In the Unzip dialog, select a target folder and click Unzip
    (Legacy menus/toolbar - Select the All files/folders in archive radio button and click Extract)
  4. WinZip will extract any undamaged files to the selected folder; for any damaged files, WinZip will display a CRC Error message. For each such file, before clicking OK in the error message window, use Windows Explorer to open the target folder and look for the file; some portion of it will probably already have been extracted. Copy the partial file to another folder and then click OK in the WinZip error message window. This will erase the partially extracted file from the target folder, leaving intact the copy of the file you made in the other folder.

Whether or not a damaged file partially recovered using this procedure 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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