The "empty USBMS" export is an expected behavior in *some* circumstances.
I honestly don't recall *all* the details, but it's not necessarily indicative of any real failure. What I do seem to recall is that trying to put stuff on it in this state will certainly be useless
.
(I *think* that's a vanilla feature of the old mass-storage Linux USB gadget, so, checking its source might yield some info, but probably won't really help you fix it
).