Yes, exposing a block device over USBMS is a solved problem. Exposing files from a device's internal storage (which is ext4 in this case) over USBMS is very mu

Yes, exposing a block device over USBMS is a solved problem. Exposing files from a device's internal storage (which is ext4 in this case) over USBMS is very much not a solved issue. You'd have to copy the files back and forth or write a driver which emulates a FAT filesystem. The alternative would be to use FAT internally and expose that block device, risking corruption (you have no control over what is done to your poor filesystem). None of my Android phones can do that. In fact, Android used to expose the raw device in earlier releases and stopped doing it because it was too unreliable and the UX was bad since the filesystem had to be unmounted.