Restore cannot validate zip file, 0x8100001A

I am trying to restore several folders from a backup on an external hard drive. The backup file appears to be empty (0 bytes), but I am certain it is not. When I try to restore some of the folders I get the following error:

> Restore encountered a problem while validating the zip file:
\\?\Volume{0f497a5d......435}\XXXX\Backup Set 2011-12-12 175633\Backup Files 2011-12-12 175633\Backup files 239.zip

The following information might help you resolve the problem:
The backup file could not be found. Make sure the drive that the backup is saved on is attached or try to restore from a different backup. (0x8100001A)
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Strangely, I am not even interested in restoring from the backup in the error message, only from my most recent one (2012-05-04).

I have not moved the location of the backup, so the additional/hidden files should still be on the external drive as far as I know.

Thanks for any assistance!

Update:
I can manually go into the backup and extract individual files from hundreds of "Backup files xxx.zip" archives. It's only when I try to restore or search for files within the backup and restore tool that everything looks empty... almost as if some index file is missing/corrupt?

In a pinch I probably could extract everything and manually delete the oldest versions of everything, but we're talking over 100gb of backups over many months, and I'm interested in restoring about 600 files... If anyone knows how to rebuild the index that would save SO much effort!!!