Hello,
Backup Exec 9.1 provides support for unlimited stand-alone tape drives. However in case of Robotic Libraries, you need to purchase the Library expansion option for every additional drive. Thus so far as you have stand alone drives, you will not need any add-ons.
In your case it seems the backup needs one more tape in order to continue with the job. You can configure the setting accordingly so that the job spans to the next available media. You can refer to the following technotes to know more about how backup exec picks a media for backup and what are the setting that can help you as per your requirement:
Title:The basics of Advanced Device and Media Management for VERITAS Backup Exec (tm) for Windows Servers
http://support.veritas.com/docs/192265
Title: An explanation of the "Overwrite Protection Period" and the "Append Period"
http://support.veritas.com/docs/237374
In case the technotes do not answer your questions or if you need more information, do write back to the forum. However if we do not hear from you within two business days, this post would be marked ‘assumed answered’ and would be moved to ‘answered questions’ pool.
Regards.