Full backups must be done within a 24 hour period. Full backups are done one day per week. There are no network problems durring backups. There are no AFS problems durring backups. One tape takes 3 hours to write.
All tapes have no errors and are the correct length.
If you have to be able to respond promptly when a customer needs data restored, you have to keep one drive idle. So it's only 224 tapes, or 448G of data.