Good morning. Please configure NAS device entry within Array Management using credentials with the proper permissions.
If not, the NDMP credentials will be used to try to access the file system but might not have sufficient privileges
Here are some documentation links for your reference:
Adding a NAS Client for NDMP Backup Operations
http://documentation.commvault.com/commvault/v11/article?p=19601.htm
Adding a Storage Array to the CommServe Database
http://documentation.commvault.com/commvault/v11/article?p=36936.htm
Editing the User Credentials or Description of a Storage Array
http://documentation.commvault.com/commvault/v11/article?p=36939.htm
Please let me know if this helps resolving the issue.