Event ID : 9539

Issue : Exchange store has dismount.

Event Source : MSExchangeIS Mailbox

Cause :
In a storage group with two or more databases, and where each database is on a separate physical hard disk, all the databases in the storage group are dismounted if one hard disk fails.This issue occurs if one of the databases in a storage group is not available.According to Microsoft, this event is logged when a store is dismounted. This can be caused by an administrative action inside of Exchange System Manager (ESM). Additionally, this could be due to a problem where the drive has filled up or the database exceeded the 16-GB size limit .

Description : Exchange store has dismount.
The Microsoft Exchange Information Store database "First Storage Group\Public Folder Store (Server_Name)" was stopped.

Action Required :
If Harddisk failed then replace the failed hard disk and restore the missing database from a backup to the new hard disk.Kindly contact On-Site Technician to solve this issue and try to mount the Exchange store.

Ref Link: http://support.microsoft.com/kb/812452