I have checked my SanDisk 32GB flash drive on ADU (should work in the same way) and with proper wiring while plugging the status changes:
Disconnected - AccesingUSB - Saving
With exchanged digital pins:
Disconnected - AccessingUSB - Disconnected (stable, not changing again)
It is not easy to figure out as there could be many possible reasons.
What models of USB sticks do you use?
Does it work when engine is turned off?
Did you connect USB cable shield to GND?
Could you provide a log file with channels under “Logger” section logged with at least 100Hz when you connect the USB stick?