You only need 2nd HDD if you have data that needs to be saved.
Or if you have data that you want to save, but can't make big enough encrypted partition to save files there, before encrypting rest of HDD
If you are beginner, I suggest you work a while with FreeBSD and unencrypted HDD until you get used to it, and When you know exactly how you want to partition your HDD, and know how to fix things if you mess up, then go for encrypting HDD.