We only support one USB attached storage device. We do this for reliability. This has nothing to do with Roon. This is a function of the sonicTransporter. You can attach other USB devices such as a USB backup drive or even a USB DAC.
We don’t recommend attaching a DAC as the audio quality would be the same as a PC or MAC and much lower then our microRendu or Sonicorbiter SE.
The sonicTransporter has multiple USB ports for all this plus future expansion.