G-Speed XL - standalone mode?

I have 2 trashcan Macs connected to each other via Thunderbolt. One of them is connected to a G-Speed XL unit. How do I make it so that when I shut down the Mac the G-Speed doesn’t power down as well?

You can’t. The device requires an active Thunderbolt connection to power the device. As soon as that connection is lost the unit will shut off.