Event ID - 9521

Event Id9521
DescriptionThe database "First Storage Group\Mailbox Store Computer Name" cannot be unmounted because Backup is in progress.
Event InformationAccording to Microsoft:

This behavior can occur if a backup process is running against another store (public or mailbox) in this same storage group. In this case, you cannot dismount another database in this storage group.
To work around this behavior, wait for the backup of the other database to complete, and then attempt to mount the database.
Reference LinksYou Receive an Error Message When You Attempt to Dismount a Database

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