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An error occurred while compressing a file, but the library was unable to remove the file's data that was already compressed and written into the zip file. This can happen, for example, when a file being compressed to floppy disks has already spanned a disk and then becomes inaccessible to the library. The library would normally seek back into the zip file, erase the already written compressed data for the file, and continue with the next file to zip. It cannot do this if a new disk was inserted.
The zip file will contain extra data that will not affect other files, but that makes the zip file unnecessarily larger. It is therefore recommended to rezip the files or pass the zip file through the convert method, to eliminate the extra data that was inserted due to this situation.