Did this behaviour start only after you tried to use USB disk mode ? Everything was working ok before ?
Only way to proceed is via a process of elimination. Could be the unit or the card, or indeed both., although that’s highly unlikely.
Some things for you to try if you haven’t already:
Seems some OT / Computer combinations like the OT to be powered up AFTER the USB connections are made.
Have you tried a different USB cable ?
Can you lay your hands on a card reader ? If the card mounts ok with that and you can copy your set to it then there could be something up with the unit or the connection/power up order.
If it doesn’t mount can you try a different card ?