So, here's what I've come up with so far......
I hooked into udev via /etc/udev/rules.d/local.rules as:
KERNEL=="sda", ACTION=="add", ENV{SYSTEMD_WANTS}+="backup_to_usb.service"
KERNEL=="sdb", ACTION=="add", ENV{SYSTEMD_WANTS}+="backup_to_usb.service"
That fires off the following systemd...