When using Agent for Microsoft Hyper-V Backup Exec's Granular Recovery Technology (GRT) does not support virtual machines that use dynamic disks, such as spanned, mirrored, striped, or RAID 5 disks.
 

Symantec Backup Exec (TM) 2012 Readme Agent for Microsoft Hyper-V notes section. (DOC521).

WORKAROUND:

Perform a Remote Agent (RAWS) backup of the guest virtual machine.

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.
 
Symantec Corporation currently plans to address this issue by way of a patch or hotfix to the current version of the software. Please be sure to refer back to this document periodically as any changes to the status of the issue will be reflected here. A link to the patch or hotfix download will be added to this document when it becomes available. 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.