The official word is posted on the FX website. It is always best to have a separate drive for the data..... That said, I have been using my internal, 7200 rpm drive on my 15" MBP for my BFD2 gigging kit for a couple of years with no problems. I only keep the gig data on the computer. It always depends on the computer.
If you are going to play live with BFD3, you may be Ok. You can still select 16bit mode for the engine. With 64 kit pieces you should be able to do an entire gig with one big preset! Use patches on the Zendrum to trigger the different instruments in the 64.
So.... It is worth a try......