A good monitoring program like SMS, MOM, or Quest’s Spotlight on Exchange can do this for you.
REGISTER or login:
f you have SCOM, it can monitor the event logs and send the alert for you. If you want to roll your own, you can use a scheduled task and a script to search the event logs for the dismount events, or use WMI to determine the mount status. You'll need to take into account the fact that if you're getting your alerts via email to an Exchange account, you won't be able to receive the alert if the store that dismounted is the one that the alert is being sent to.
Here's a Powershell solution that you shoule be able to set up to monitor the event log on the server and email you when the dismount events occur.
Powershell Centralized Log Monitor
Hope this helps.