Unrar Multi-Volume Extraction Behavior Explained

When extracting a file that spans across three distinct RAR volumes, the unrar utility processes the data as a single continuous stream, reading each volume sequentially until the file is fully reassembled. This process involves automatic volume detection, continuous stream writing to the destination disk, interactive prompts if a volume is missing, and a final integrity verification once the file reaches completion in the third volume.

Sequential Volume Traversal

To begin extraction, you point the unrar command to the first archive in the sequence (such as archive.part1.rar or archive.rar). The utility reads the archive header, detects the multi-volume split flag, and begins decompressing the file from the first volume directly to the target destination.

When the decompressor reaches the end of the first volume, it does not close the output file. Instead, it reads the continuation flag embedded in the volume's end block, identifies the exact file name required for the next segment (e.g., archive.part2.rar), and automatically opens it if it resides in the same directory. This transition occurs entirely in memory; the partially extracted file remains open on the filesystem while data from the second volume continues appending to it. The exact same transition takes place moving from the second volume to the third volume.

Missing or Relocated Volumes

If unrar cannot locate the second or third volume automatically in the current working directory, it halts processing and prompts the user via the terminal:

Cannot find archive.part2.rar
Insert disk with this volume and press Enter or enter the correct path:

The process will wait indefinitely for user intervention. If the correct path is provided, extraction resumes without loss of previously extracted data. If the extraction is aborted manually or terminated due to an error, unrar by default deletes the incomplete output file from the disk to prevent corrupted data from remaining, unless the -kb (keep broken extracted files) switch was explicitly enabled when running the command.

Checksum Verification and Finalization

A file spanning multiple volumes contains only one master checksum (CRC32 or BLAKE2) stored across the split set. Because of this architecture, unrar cannot verify the integrity of the individual segments within the first or second volume on a per-file basis.

The integrity check occurs only after the final byte of the file is decompressed from the third volume. At that point, unrar calculates the total checksum of the output file and compares it to the original checksum stored in the archive headers. If the checksum matches, unrar applies the original file attributes and timestamps, closes the file handle, and marks the extraction as successful. If there is a checksum mismatch, unrar alerts the user to a CRC error and deletes the file unless instructed otherwise.