Det går inte via FireWire alls:
”macOS Catalina will not boot from a FireWire device
Apple has dropped support for booting from FireWire devices. The macOS Catalina Installer will explicitly disallow installation onto a FireWire-attached device, and if you attempt to boot macOS Catalina from a FireWire-attached device, the startup process will fail with the universal "no entry" symbol.
Solution: If your external device also has a USB interface, attach the device to your Mac using a USB cable instead.”
https://bombich.com/kb/ccc5/macos-catalina-kn...