An error occurred during backup and restore. The backward-compatible document library cannot be backed up.
To resolve this issue execute the following steps:
Look in the Application event log for events and information related to the content index. The Application event log can be viewed through the Event Viewer snap-in which can be found in the Computer Management console. The following task will start the Computer Management console:
Start Computer Management Console
The resolution depends on the error. The event should provide guidance about the type of error. Typical errors include:
Permissions. An account that is used during the backup process does not have the necessary permissions for the file share. The accounts for the SQL Server, IIS Application Pool, SPS Backup Service could be involved.
Hard disk problems