Same issue here with the OT MKii. Powers on in USB disk mode, samples dropped into OT folder on macOS. Then they do not show up in OT (audio pool still only shows samples from the card (not those added in USB disk mode.
Shortly thereafter OT freezes and Mac advises that disk was not ejected properly (disk was never ejected or units powered off…)
Tried several work arounds, same issue.

Thanks in advance for any advice!