Media Type 4mm DAT320 is not listed within BE 2010 and BE 2012.
The media type can be found under the "media type" tab of the tape drive properties.
Backup Exec 2012
A hotfix is now available for this issue in the current version(s) of the product(s) mentioned in this article. Refer to the Hotfix link under Related Articles at the end of this article to obtain the hotfix needed to resolve the issue.
Backup Exec 2012 Rev 1798 Hotfix 201596. Device Driver Installer DDI20121215 (December 15th, 2012) (TECH201596)
Backup Exec 2010
Symantec Corporation has acknowledged that the above-mentioned issue is present in the current version(s) of the product(s) mentioned at the end of this article. Symantec Corporation is committed to product quality and satisfied customers.
There are no plans to address this issue by way of a patch or hotfix in the current or previous versions of the software at the present time. However, the issue is currently scheduled to be addressed in the next major revision of the product. Please be sure to refer back to this document periodically as any changes to the status of the defect will be reflected here. Please note that Symantec Corporation reserves the right to remove any fix from the targeted release if it does not pass quality assurance tests. Symantec's plans are subject to change and any action taken by you based on the above information or your reliance upon the above information is made at your own risk.
Please contact your Symantec Sales representative or the Symantec Sales group for upgrade information including upgrade eligibility to the release containing the resolution for this issue. For information on how to contact Symantec Sales, please see http://www.symantec.com