A storage device is in the block subsystem, so you'll want SUBSYSTEM=="block" in your rule, like this:
ACTION=="add", KERNEL=="sd?", SUBSYSTEM=="block", ENV{ID_BUS}=="usb", \ RUN+="/path/to/script"
If you're using systemd, you could run a systemd unit each time a USB storage device is added. Create the unit file, e.g. /etc/systemd/system/my-usb-rule.service:
[Service] Type=oneshot ExecStart=/path/to/script
and the rule, e.g. /etc/udev/rules.d/85-my-usb-rule.rules:
ACTION=="add", KERNEL=="sd?", SUBSYSTEM=="block", ENV{ID_BUS}=="usb", \ ENV{SYSTEMD_WANTS}="my-usb-rule.service"
Now udev will trigger my-usb-rule.service (which in turn will execute your script) on any usb storage device add event.
Don't forget to reload the configuration after you edit the rules/units:
udevadm control --reload systemctl daemon-reload