How can I monitor if an external USB drive gets disconneted from a server?

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http://serverfault.com – I have a few servers that rely on external USB drives for some of their backup scripts. However, there have been times when the USB drive might get disconnected accidentally (upon reboot or power outage or just anomalies). This causes the backup script to fail to run. Anybody know of a way to monitor if a drive is available and send an alert (SNMP, email, custom event log entry, doesn't matter) if it isn't available? Something that polls on a regular basis (batch file, SNMP trap, etc.) To keep this from being a "shopping question", I'm just curious what WMI or SNMP counters I can query ag (HowTos)