About CRC errors
A CRC error indicates that some data in your Zip file 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 telephone line noise or some other transfer error when downloading a Zip file from an online source such as the Internet or AOL. Such an error can introduce invalid data into a Zip file. Some other possible causes include exposure of disks 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 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 copy of the disk or Zip file that is undamaged. There are, however, two other situations in which it may be possible to recover some files from a removable disk.
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.
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.