Clients with external harddrive



I have some (Windows) clients, which have, sometimes, a external harddrive connected. I would like to backup these external drives, too. It would be create if a client could register an external device to be added to the backup. It should be done not by drive letter, because this can either change or be a total other media (e.g. USB Stick). It would be good if the drive is recognized by serial or some kind of id.
Can this already be done somehow? If not it would be nice if this feature would be added.