I have had this happen and narrowed it down to a track not having the data analyzed
go to the usb drive area in RB and see what the last thatch that actually WAS exported is and then see what the next one should have been in the main collection, thats the easiest, fastest way
if you can't, then select ALL the tracks on the main collection and re-analyze them all to confirm everything has a waveform and then format your usb drive and export again, maybe try 1 playlist at a time as a test to narrow down what song in what playlist may be causing the issue
this is where I miss 2.x also because at least that version showed you what tracks were currently being exported