USB ISR + SPI

Seriously? I'm aware that you probably see your share of stupid questions on a daily basis but being a cheeky arse really changes very little and only serves to drive people away. I've posted here before and have read the stickies. It was either post in programming questions or in project guidance and I opted to post here because it was a general programming question that others doing USB communications work with peripherals might have as well. I am attempting to ask a question to prevent more nefarious errors later on. I don't think anything I've done here is unreasonable.