Validation of diff area files completed. Number of Diff Areas: %2 Guidance: When a volume is mounted, the volume snapshot ...

Validation of diff area files completed.

Number of Diff Areas: %2

Guidance:
When a volume is mounted, the volume snapshot driver reads and validates all the diff area files located on the volume.  These diff area files may be for persistent snapshots of the volume being mounted, or for persistent snapshots of other volumes.

You should expect this event when mounting a volume.  No user action is required.