It likely is a byproduct of setting the device to run as a AP. When you use a router as a AP you lose many of the feature since they are dependent on traffic flowing between the wan and lan interfaces. When you run in AP mode the lan and wan are bridged together. In this case since the USB drive normally is visible to the lan it may just a be that they disabled this feature as part of the AP mode.
I would run the device as a router and connect via the lan ports. You will need to do all the work the command does like disabling DHCP.