Android usb host api and USB storage

The Android USB Host APIs do not include USB Mass Storage filesystem code, nor as of this writing in 2014 will "stock" Android mount a USB Mass Storage volume at operating system level.

To access a USB Mass Storage device using the stock Android USB Host APIs, you must therefore implement in your Application code both the necessary raw-USB operations to achieve block-level device access, and the appropriate filesystem logic itself. Needless to say, the details of such are of a complexity beyond the scope of an answer here, but you could start by studying documentation or existing implementations of USB Mass Storage drivers and filesystem drivers for other platforms.

It appears the situation in Android 6 may be different, and access via the USB host apis to something that version recognizes as a storage device might even no longer be permitted. Those targeting Android 6+ may need to look elsewhere, but older devices will remain in use for some time.