Friday, March 23, 2012

When a Zip is Not a Zip

For a while I administered a department-level "server" (basically, a large and noisy workstation) running some flavor of Windows (I think Win 2K).  Colleagues in my department used it for assorted purposes, including research (web-based surveys), and like all good users they (a) never deleted anything from the server (since, from their perspective, space was free) and (b) were cavalier about maintaining their own copies of their files. When the time came to retire the server, I sent out a couple of warning emails (get your files or wave goodbye to them).  I also created a ZIP archive of all data files on the server before turning it into a doorstop.

Recently (meaning a couple of years or so after the server was recycled) a colleague asked me if I could recover her surveys and survey data.  So I scrounged around and found the ZIP archive ... only to discover that it cannot be decompressed by any program I can find.  I've tried at least seven alternatives, including the compression/decompression method built into Windows 7, several popular third-party programs for the Windows platform, and gunzip on Linux.  They all show the contents of the archive correctly, and they all fail (either with an error message or, in one case, by freezing) when I try to extract anything from the archive.  The error message is that the compression method (described by the ordinal 18 in some cases and the name "IBM/Terse (new)" in others) is unsupported.

Up to now, I've naively assumed that a ZIP file is a ZIP file is a ZIP file.  I know that what's being zipped could be a TAR archive, a bunch of files or whatever, and I know that some ZIP utilities give the user options for trading extra time for smaller file sizes.  I've never had an uncorrupted ZIP file fail to decompress using any compression program, whether the same one that created the archive or not ... until now.

In the future, I guess I will need to be extra careful about how important backups are compressed -- and hope that the compression method used does not go the way of the card reader.

12 comments:

  1. Maybe it is Zipx (WinZip file format) or something similar. Did you try extracting it with 7-zip?

    Best regards,
    Thomas

    ReplyDelete
  2. Thanks for the suggestion. Yes, I tried 7-zip; also WinZip, the PKWARE zip reader, IZArc, a PC port of Terse (tersepc), the built-in zip/unzip capability in Windows 7, and gunzip. No joy with any of them.

    ReplyDelete
  3. Do you know which software you used to create the archive?

    Best regards,
    Thomas

    ReplyDelete
  4. Unfortunately, no. I think perhaps the built-in zip functionality in Windows Server 2000, but I'm not sure.

    ReplyDelete
  5. As far as I know, Windows 2000 does not have an internal ZIP tool.

    As you can see the file contents, just look into the Program Files folder (if you backuped it) to see which archive tools were installed.

    Best regards,
    Thomas

    ReplyDelete
  6. Thanks for the suggestion. Alas, I only backed up data file, not program files. It's possible I copied the files over to my (then Win XP) desktop and zipped them there, in which case it would either have been the XP native compression method, PKZip (which I know had at one time) or one of the other common free zip tools (such as WinZip).

    ReplyDelete
  7. Hmm, I'd really like to help you but I do not have any further ideas. I found the page http://en.wikipedia.org/wiki/User:Closeapple/info/Comparison_of_ZIP_file_archiver_support that claims that no ZIP tool has implemented TERSE (18).

    Did you try to extract other files (every file can have its own compression)?

    Do you know someone who has PKZip? I would not expect Windows native compression to do such strange things.

    Best regards,
    Thomas

    ReplyDelete
  8. Thomas, I truly appreciate your efforts. I've tried extracting individual files with no luck. The closest I came (this morning) was with ZipGenius. It did not give me any error messages relating to TERSE, but it did give me a checksum error on every file that I tried to extract. As far as PKZip goes, they now have a free ZIP reader program, which I downloaded and tried. It gave me the unsupported compression method message.

    My new thought is to write the archive to a thumb drive, put the thumb drive on an anvil, and smack it hard with a hammer. :-)

    ReplyDelete
  9. 2 more ideas:

    I found this tool: http://www.diskinternals.com/zip-repair/
    Maybe it might help you.

    Did you try extracting the archive using the Windows XP tool. I think I read that the Windows 7 tool lacks some features that the XP tool has.

    Best regards,
    Thomas

    ReplyDelete
  10. This comment has been removed by a blog administrator.

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  11. One last footnote to this: I never did find a way to unzip the archive, but I found a different backup copy of the same files that had not been compressed, which solved the immediate problem without answering the puzzle.

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  12. You should try following tool it will help you zip repair program restores corrupted .zip archives by reason of crc errors, corrupted zip file from Internet - your case, viruses and so on.

    ReplyDelete

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