full disk encryption only protects you from passive snooping. If someone has physical access between two of your subsequent uses, no amount of any type of encryption will save you. Except maybe some entangled quantum bit collapsing mechanism. Maybe.

Think hardware keyloggers, fake MBRs, &c.

OP's trick won't work, but that's an "implementation detail;" there are plenty others that will.

EDIT: to clarify; that's not what you said, it's just a common enough misconception that it's worth being explicit about, here.