I assume that this is a calibre question -- that you are seeing this prompt in calibre. My guess is that in calibre you changed the default send to device behavior. If you click the down arrow on the send to device button then choose "Set default send to device action" and choose "Send specific format to" ..., calibre will ask you every time which format to send. To make that go away, choose "Send to main memory".
This old calibre thread goes through the issue in excruciating detail.