And one other idea, assuming that you are collecting QoS for the existing and new drives, it would be possible to create a list of the existing known source/target combinations from the s_qos_data table and then alert when this table changes. This would also let you detect when devices were removed too if you filtered the list for QOS received in the last 24 hours. Might be able to run the query against the qos_snapshot table too.
Or, and it would take some investigation, but data_engine will record creating the QoS entry in its log so you could process that with logmon.
Many different ways to skin the proverbial cat here.