Expose folder/partition on internal storage as USB peripheral mass storage device

[Doug Kadlecek]

I am trying to create a way for a user to copy a file directly to a device without seeing the entire file system. Ideally, I would expose a FAT32 partition on internal storage and when the device s connected to a computer via USB cable, a user could see the partition and write files to it (aka a USB Flash Drive).

Is this possible? It's unclear to me how this would be setup via AOSP (note we are using Android 6 as our base for the device). Would this be accomplished at the Linux level? We also would want to disable MTP, but I already know how to do that.