Thanks for your replies. Since the advent of mirror backups I realised there was an opportunity to backup to several different USB disks where each disk is a different remote allowing rotation of the disks. The primary backup goes to additional internal storage which is then mirrored to the USB disks. Only one snapshot per vm is needed regardless of how many USB disks are in use. To do this I need to script the mounting process for these disks and send notification emails when a disk is mounted, unmounted (safe to unplug) or when no disk could be found.
This is my use case.
Thanks
F