Microsoft Knowledge Base Article 244805 contains the following summary:
When trying to restore from a backup set you may encounter the following error message:
This set cannot be
cataloged because the media containing the set data is missing.
Windows 2000 Backup (Ntbackup.exe) does not have a /missingtape command-line switch that you can use to restore data from an incomplete multiple tape backup set. For more information about the /missingtape command-line switch, refer to the following article in the Microsoft Knowledge Base: 101228 Backup: /MISSINGTAPE Command Line Switch.