A Sharepoint agent tape backup with GRT enabled where the Sharepoint SQL server uses a hardware VSS snapshot provider mail fail with access denied.
 
Job Error
Access Denied. Cannot backup directory and its subdirectories.

Workaround: 
Edit the backup job settings for the Sharepoint farm resource to change the Advanced Open File Options to use the 'Microsoft Volume Shadow Copy Server (Window 2003 and later)' selection with the dropdown set to 'System - Use Microsoft Software Shadow Copy Provider'.  This will prevent the backup job from the using the Hardware provider for snapshots.