What Does a CRC Error Mean in Unrar?

A Cyclic Redundancy Check (CRC) error when running unrar indicates that the data inside the archive has been corrupted or altered, meaning the extracted file does not match the original file created by the archiver. This guide explains the mechanics of a CRC check, the common causes of corruption during extraction, and the practical steps you can take to repair or bypass the issue.

Understanding the CRC Mechanism

When a RAR archive is created, an algorithm calculates a unique mathematical checksum (a hash value) based on the exact binary content of each file and stores it in the archive header. When you run unrar to extract the archive, the utility recalculates the checksum of the unpacked file and compares it to the stored value. If the two values do not match, unrar aborts or alerts the user with a "CRC failed" error to prevent the use of damaged data.

Common Causes of CRC Errors

  • Incomplete or Corrupted Downloads: Network drops, packet loss, or server-side truncation during download often lead to missing or scrambled bits inside the archive.
  • Storage Drive or Hardware Issues: Bad sectors on a hard drive, failing flash memory, or faulty system RAM can corrupt the archive while it is stored or while it is being read into memory.
  • Multi-Part Archive Issues: If you are extracting a split archive (e.g., .part1.rar, .part2.rar), a single corrupt, missing, or misnamed volume will cause a CRC failure for any file spanning across those segments.
  • Incorrect Password: While newer versions of RAR typically notify you of an incorrect password immediately, certain configurations or legacy formats may attempt extraction and fail with a CRC mismatch instead.

How to Fix or Bypass a CRC Error

  1. Re-download the Archive: Because the most frequent cause is a faulty download, clearing your browser cache or download manager and downloading the file again often resolves the problem.
  2. Use the Built-in Repair Feature: If the archive was created with recovery records, you can attempt to repair it using the command: 
    rar r archive.rar
    This generates a fixed.archive.rar file that reconstructs the missing data sectors.
  3. Keep Broken Files: If the archive contains media files (such as video or audio) where minor corruption is tolerable, you can force unrar to retain the extracted file rather than deleting it upon failure. Use the -kb (Keep Broken) switch: 
    unrar x -kb archive.rar
  4. Verify Storage Integrity: If CRC errors occur frequently across different files, run diagnostic tools like chkdsk (Windows), fsck (Linux), or MemTest86 to check your storage media and RAM for hardware failure.